ReaVerb is primarily an impulse reverb. The "echo generator" can be used, but is about as limited as any other single reverb algorithm, and is not ReaVerb's strongest point.
Impulses are basically wavefiles recording a perfect pulse (or something close, like a popping balloon) in a reverberant environment.
Download some from
NoiseVault, save them on your hard disk (in their own folder, preferably).
In ReaVerb, click on "add", and select "file". Now browse to one of your reverb impulses and load it.
ReaVerb will now apply the reverb of that space to whatever is ran through it.
Stay away from the thirty-second long Taj Mahal and cistern impulses until you've messed around a bit, though.
ReaVerb's other tools allow you to build your own echoes and such, or process impulses, but the impulse function is the main one.