Regarding the schematic, the PDF viewer built into Firefox seems to be having a lot of problems. Try downloading the file and viewing it using Acrobat Reader or another PDF viewer on your computer. This should answer some of your questions.
You’re right that there should be more labeling on this board than there is. This board is meant to be an Arduino “Shield” which has common pins in fixed positions. You can certainly use it with a PIC or any other microcontroller though. I’ve put up a picture showing the positions of the pins you need at: http://dlnmh9ip6v2uc.cloudfront.net/datasheets/BreakoutBoards/MIDI_breakout_pinlabels.png. We also supply the EAGLE files for this board. EAGLE is the software we use to design all our PCBs, and is available from www.cadsoftusa.com. If you install EAGLE and load the board file, you can see which lines go where.
Sorry for the confusion, I hope this helps you get up and running.
EDIT: This board is meant to run on 5V.