How do I get GB'11 to recognize my digital piano as a MIDI keyboard through an audio interface?

I have a Tascam US-144 MKII Audio interface that has a MIDI IN/OUT along with several other mic/line inputs. I have a digital piano that has a MIDI IN/OUT that I would like to use as a MIDI keyboard for the "Software Instrument" track to use the MIDI instruments via GB. I cannot figure out for the life of me to get this little operation going. I have ran mics, guitars, and even a MICROKORG through the interface and successfully recorded with them, but cannot seem to get garageband to recognize a MIDI keyboard when I have the digital piano hooked up to my interface. Can anyone help me?!