When you’ve missed installing the GarageBand Extra Content (I.E. “GarageBand Instruments and Apple Loops”) for whatever reason,
and when GarageBand DOES NOT give you the option to install from DVD when clicking on a non-installed loop/synth etc, then you can try the following. YMMV
Note: iLife 11 doesn’t seem to have the extras on the DVD (I could have missed it though)
So, with that in mind, break out your MAC’s Applications installer disc and pop it in and download the attached instructions.