Here's an addition to the above suggestions. Quickest way to get a synth sound into a loopable clip lickity-split is to:
a) record a measure of the sound you want (hold down a note on your keyboard or controller, or just click add a note in the piano roll). Advantage here is that you can pick the note to hold down (if the synth patch is tunable) for a higher or lower sounding result.
b)bounce the track (tracks|bounce to track in track view) and voila! (french for "flower") you have a editable clip of the sound you are after sitting on a track.
c)edit the clip to your heart's content. Chop it, trim the ends, acidize it, loop it, whatever.
d)open the media browser and drag the _fixed_ clip to the folder where you want to save it and, again, "voila!" ("flower!") it's saved and ready to use. You can change it's name from "clip.wav" to something more useful, like "voila.wav"?
(It took me way longer to type these instructions than it would to perform the steps--AND drag the new clip into a matrix view column to play with it.)