Umm, let's see. The easiest way to do it is with the apple loops utility that comes with Soundtrack pro, but if you don't have that...
Open Garageband, create a track, grab one of the loops you want, drag it right onto the track...that's one way. you can then select, "add to loops library" from the very top options.
Also, try converting all the wav. files to aiff in iTunes..
Then take the newly made aiff loops, and drag them into the loop browser in Garageband. The actual loop browser thing, not the track..