I just plugged my Roland A-49 to my MacBook Pro (Early 2011 2.3 GHz). The MIDI keyboard would have its power supplied via USB, but it has no power. The specification says it needs 98 mA current, which is way less than the 500 mA provided by the MBP's USB 2.0 port. If I plug it into a desktop PC, it works. What can be the problem? An USB hub would solve this issue, I suppose, but can I avoid buying one?
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.
You will need to disenable the SIP control of the Mac OS. Here a link that tells you how to do that; http://osxdaily.com/2015/10/05/disable-rootless-system-integrity-protection-mac-os-x/