If you are not able to trigger an external synthesizer or sound module connected to the Keystation Pro 88's MIDI Out port, either from the keys or from a computer sequencer, it is most likely because you do not have the MIDI Out from USB function properly set.

The Keystation Pro 88 can interface with other MIDI devices.  By default (i.e. when you switch the unit on), all controller data is sent out via both the MIDI output and the USB output (to the computer).  For the MIDI output to function as a USB-to-MIDI interface, press the MIDI Out from USB buttons (PROGRAM + DATA LSB) to activate the MIDI Out from USB mode.  If you are using a MIDI host application that can pick up multiple input drivers, you will be able to use the Keystation Pro 88’s MIDI input and the Keystation Pro 88 control surface to record MIDI data and send it out via the Keystation Pro 88 MIDI output.

Note: The Keystation Pro 88 is not a MIDI Thru device. MIDI data received at the Keystation Pro 88 MIDI input can never be sent directly to the Keystation Pro 88 MIDI Out port. However, if the Keystation Pro 88 is connected to a computer via USB, data received via the MIDI In port can be transmitted to the MIDI Out port because the data is sent to and received back from the computer.  MIDI Out From USB mode must be engaged for this to occur.