Well if I answer this one, it's going to be overly verbose with a risk of references to the block diagram...
but if you're ok with that, here goes...
There are 2 options for sending the track signal to your reverb box, depending on whether it needs a mono or stereo signal on its input. The simplest way is to use mono as you only need one cable. I'm also guessing it will generate a stereo reverb from that single input. If not, I'll describe another option at the end.
To send a signal out of the machine, you can use either of the Effect Send sockets at the back of the machine. Send-1 is also connected to the internal FX, so although you can still use this for external effects, it can be easier to just use Send-2.
So, how to get the signal from the track to Send-2...
Press Mixer, then press the Select button on the reqd. track. You'll see that track's number appear on the top-right of the screen - this means everything you see on the screen applies to that track, and only that track.
Use the cursor buttons to move the highlight area over the Send Eff-2 On/Off box and set it to Pre or Post. Normally you'd use Post for a reverb effect. Make sure Send Eff-1 is Off.
The Send-2 knob and the Master-2 knob both control the signal level which will be sent to the Send-2 socket. You can check this signal by pressing the Monitor Select button and choosing the Effect Send-2 option. When you press Play, the signal from your chosen track will come through your monitors - if you used 'Post' you can also control the amount via its fader. Adjust the knobs to some arbitrary value just so you can hear something - you can tweak them later and/or the track fader to adjust the amount of reverb reqd.
Don't forget to set the Monitor Select back to Stereo afterwards.
The reason there are 2 knobs is that the Master does what it says and controls the send levels from all tracks and inputs. Each track and input has its own send level, so you can have some high, some low etc. but the combined send signal then goes through the master control so it overrides everything, maintaining the relative levels. If the master is zero, then there will no send signal for internal or external effects.
Connect a cable from the output socket called Effect Send 2 to the input on your reverb box. If it has a left & right input, just use the left one for now.
How to get the reverb signal returned to the machine...
Assuming your reverb box is generating a stereo output, connect 2 cables from its left & right outputs back to 2 inputs on the dp24.
Not sure what your requirements are as you ask if it needs recording. If you're mixing then no - the return signal can be routed straight through to the stereo bus (via the Assign screen) and then into the mix. If not, you can record it to a spare track and then use this track later when you're mixing.
So if you want to record the reverb to a track, arm the track, set the Trim levels for the 2 inputs as reqd. then press Record and let the song play through. All other track faders are irrelevant as the Send signal is only coming from the track you set up at the start.
During this phase, you can alter the reverb amount on-the-fly by tweaking the track fader or the Send levels. If you use the latter make sure the track is selected first otherwise you'll be adjusting the wrong track. You won't be able to get into the Mixer screen so there's no confirmation of the track in the top-right. As soon as you turn a knob, you'll see a pop-up screen showing just the Send levels - it will disappear automatically.
When the song has finished, press Stop and dis-arm the recorded track.
If your reverb needs a left & right input to generate a proper stereo reverb, then...
You need 2 signals from the dp24, so you can use Send-1 to send the left, and Send-2 to send the right. If you're wondering if you should pan the signals first, you can see from the block diagram that the pan controls are on a separate path to the send signals so the answer is No.
You just repeat the above for Send-1, i.e. set Eff-1 to Post, set the Eff-1 level, connect a cable from Effect Send 1 to the right input of your reverb box.
The only slight complication is that Send-1 also connects to the internal FX, so you need to ensure this is turned off... i.e. press Effects. If the F1 button is labelled 'Send', press it to get to the Send Effects screen, then press On/Off (F4) as reqd. to turn it off. There's only one internal Send effect so you don't need to cycle through the options with the Select (F3) button.
Two more options.
Some reverbs have a wet/dry mix control where you can mix the original signal with the reverb'd signal (the dry & wet signals). If you mix them in the reverb box, the recorded track will contain that mixture, in which case you don't want to use the original track in your final mix. However, you could turn that control so that it generates the wet signal only. This means you could then control the wet/dry mixture during the final mix by using the faders on the original (dry) track, and the newly-recorded (wet) track.
So apologies for the ramble - not sure how much detail you really wanted, but hope some of it helped.
