SPDIF stands for Sony Philips Digital Interface. Basically, it is a standard for transmitting digital information. The SPDIF input on your MBox is there to allow you to use an external analog to digital converter if you have one of higher quality than what comes in the MBox.
You can record MIDI tracks on your MBox using a second interface. For example, I have a MIDI keyboard that uses a USB interface to get the MIDI information into the computer. From there, I simply set up a MIDI track in Pro Tools to record the MIDI information. You can do tons of different things with the MIDI track - trigger a software synthesizer, a drum machine, or a hardware synth, for example.
Hope this helps,
P.S. All of this should be covered in the manual that came with your MBox.
P.P.S. I'm not trying to be a jerk, but please proof-read/spell check your posts.