Hi Boo.

I dont have i30, but MicroArranger, and don't use midiplayer, but I guess you have to use the 'Song' button to find the midifile, and then 'Song Play' button to start the player.

Here is a quote from the userguide / manual page 2.

SONG PLAY key
In Song Play mode you can playback Standard
MIDI Files that were loaded from floppy disk or
hard disk (if installed).

The manuals can be downloaded from here.
http://www.korg.com/us/support/download/manual/1/242/1717/

I'll try attach players guide here as pdf, and the zipfile as is where all documents is inside. Hope this is for any help.
Good luck to reinvestigate the i30.