I use AU 3.0 but it's most likely set up the same way as yours.
Open your Mixer (using the "window" tab...then click on one of the little arrows pointing to the right that are adjacent to the blank fields in the second section down from the top of each track column. This will open up a drop-down..giving you options to Plug-in an effect. Go down to REVERB..then over to STUDIO REVERB (it's the best) and click it.
Now everything on that track in the multi-track view will have that plug in.
Play around with the PRESET reverbs by clicking the down-arrow to the right of the big blank field at the top of the Plug-in page till you find one you like..then fine-tune its settings till you get the decay, room size etc you want and SAVE it by clicking the little icon just right of that down arrow I mentioned above..title it and away you go. Every audio clip on that track will be reverbed. Plus you can add numerous more effects to that track by going thru the same process, using the other blank fields below that plug in (which now shows up in your mix window on the track wou added it to.