On already-recorded tracks.... you can either *insert* the guitar effects or you can route them to the "send" effects (or both) - the latter sounds like what your wanting to do.
Check these things first if you're not hearing the send effect:
1) Make sure you're set to either "pre" or "post" when turning on the Send 1 routing switch in the *Track* mixer screen for the track your wanting to add the affects to. For these purposes I suggest going with "post".
2) Max out the send 1 master level (this level setting is global across all tracks)
3) Max out or nearly max out the Send 1 level control - it really needs to be up around 80-90 or higher before you can obviously hear it
4) Make sure the effect is turned on by going into the effect screen, turning the effect "on" (setting parameters become colored), and **all important**: make sure the send effect "Return" level is turned up. Turn it up high at first. Once you can hear the effect - these different level setting can be adjusted to your taste, liking, preference, etc
Now you should have the effect applied to the track(s) of your choosing.
Keep in mind that the effect will not be printed. You'll only hear it applied to the stereo bus for playback. Printing the effect to the track is a whole other ball of wax.