Applies to: Impulse
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.
On the Impulse select the Setup button and ensure transport is set to "MC".
The MIDI messages in GarageRemote should be set as follows (double-click on each to edit and then type the messages shown below into the box):
From the Listener page in GarageRemote make sure you have clicked on the Start button for the transport controls to work.