Check, if your loops are properly installed.
In GarageBand '09 you usually will find your Symphony Orchestra Jam Pack Loops installed in
/Library/Audio/Apple Loops/Apple/Jam Pack Symphony Orchestra/
see this link: http://support.apple.com/kb/HT1045
If the loops are there, but GarageBand does not recognize them, try if rebuilding the Loop Index will help:
The procedure is described here: