No power through USB for Roland A-49
I do not think it is a power problem.
Check in GarageBand if your device is showing up.
If not: I would start with getting the latest software for your MIDI device
http://forums.rolandclan.com/viewtopic.php?f=54&t=45738 On my A49 you have to hold down the "Function" "Pitch" and "Assign" buttons on the keyboard all at the same time as you power it on.