This is what I found on mine - when I press Effect, it brings me to my Reverb settings screen. If I press Select (F3), it toggles between Reverb, Chorus, and Delay. On the other hand, if I press Guitar (F1), it brings me to the yellow tab labeled Amp, where I can select between compressor, noise suppressor, etc. Pressing the F1 tab (now labeled Send) returns me to my reverb settings.
So, when you're at the yellow Amp tab, try pressing the F1 key, which sould be labeled Send, and it should bring you to the blue tab, where you can select between reverb, chorus, and delay. I soloed a track as I switched between the reverb, chorus, and delay, and the effect on the track did change accordingly. Once you get to the reverb tab, play an individual track using the Solo key, and you can really dial it in exactly where you want it.
Hope this helps!
(Edited to add) Also, here is an overview of the Effects module from the official Tascam DP-24 training DVD by David Wells. He put a number of videos on You Tube to demo his DVD (which is very good, by the way - I do have it), and this one is just what you're looking for: