The MTPAV defaults to only responding to MMC from midi port #7 (as well as USB , serial or parallel ports). When the external midi device is connected to midi port #7, the MTPAV should play, stop, locate, etc.
If this doesn't work right away, set the MTPAV is at it's factory default (see topic MTPAV reset/initialize) . You can also try different MMC device id's on both the MTPAV and the MMC controller.

This setting can be changed with Clockworks or the ExpressMTP console software. Any and all of the midi ports can be configured to respond to MMC. This is not changeable in stand alone mode.