Here are a few things that caused me to have a little head-scratching on my journey to an MB-6582. The documentation from the parts and the base PCB construction guides is great but after that the hand-holding dies down a bit. So here are a few things I had to look up ... maybe this will save someone the trouble.

Finding the wiring schematic for the LCD should be high on your priority list after the base PCB is done. If you zoom in, you can see a copy of it here in the first figure of page 1 on the right.

http://www.midibox.org/users/jim_henry/building_a_midibox_lcd_cable.pdf

This is J15 on the core and the pinouts are labeled (pretty microscopically but they're there). Note all the data pins (D0-D7) are on the left column of J15.

Once you get a working LCD, download the MB-6582 application from here.

http://www.ucapps.de/mios_download.html

The one I used was midibox_sid_v2_0_rc35.zip but this may be out-of-date after a while.

Install it with MIOS Studio. A walkthrough's here:

http://www.ucapps.de/mios_bootstrap_newbies.html

I actually corrupted a PIC by stopping the application halfway through. The symptom - the PIC kept asking to upload by sending the upload request sysex string over and over every minute or so. Solution: back to the downloads and got the MIOS file mios_v1_9g.zip. Uploaded it with MIOS Studio. It was normal after that (LCD says MIOS V. 1.9f, then READY). Re-uploaded the MB-6582_setup.hex file and that was it. Success means it says Midibox Sid v... after the MIOS part.

If you have other troubles at this step, might be worth checking out

http://www.ucapps.de/howto_debug_midi.html