Wow, I can't believe this is posted today.
Just last night, I installed the same JamPack and had the SAME problem..weird!
Anyway, I've got your solution...
I found all of mine in the Garageband instruments section, when selecting a virtual instrument. (Stereo->Logic->GB Instruments). The reason I looked here is because, for pretty much all the documentation for the JamPacks, it says one of the requirements is GB. Didn't really say anything about LE or Pro, so I started there, with GB.
Pick an instrument, like "Tuned Percussion" from your GB Instruments. Now go up to the arrow right below the Bypass button on your instrument window...way more options now! I love the sound of the Timpani. Check out the other instruments, there are a bunch added for strings, and I think 2 new pianos (Steinway being one, unless I'm mistaken and it's been there all along).
I think that's pretty much it, outside of the loops. Let me know if that helps.