GarageBand offers limited support for MIDI control. In fact it only allows you to control the level and pan of the currently selected track. It is possible to use a 3rd party bit of software, GarageRemote (available to download here), to allow MIDI control of the transport in GarageBand too. We have created a template for the ReMOTE SL which allows level, pan and transport control over GarageBand. It is available in the 'ReMOTE SL Templates' download from the ReMOTE SL Software Downloads page and comes with full set up instructions.