You MUST add it under External Devices in Audio Setup.
In the configuration options, I suggest checking Default device (unless you're running more than 1 MIDI device) and uncheck Release Background Audio (that way you can jam along with YouTube videos, etc.).
Also, you have to add Instrument tracks for MIDI devices and you should be able to play any of the instruments in Presence XT.
I use a Roland A-49 with Studio One 4.1 plugged directly into a USB port on the PC and it works great. I did have to install the Roland USB driver first. If I want to use a MIDI cable to connect my A-49 to my PreSonus Audiobox 96, then it requires I use a USB hub to supply power to the A-49. I'm not familiar with how the Roland A-88 is designed to get its power and talk to the PC/Mac but I hope my info helps you get it working.