all you have to do is to install the AU (Audio Unit) Version of the MMV
Garageband only works with Audio Units, not VSTīs.
Now open Garageband, create a new Track (Software Instrument),
and the Moog Modular V now should be available in the Instruments list.
(klick "Pianos and Keyboards" on the left, the MMV should show up on the
right list of the Instruments Window.)
Now you can play and record the MMV in Garageband via Midi keyboard,
or the with the Garageband Keyboard-Window.
To open up the full MMV-Interface in Garageband:
Double-klick the Instruments icon on the left side of the track,
open the "Details" Lane and klick the little Button with the pencil symbol
on the right side of the "generator" and "manually" popup-menus.
Hope that works!