OK I did this and it worked great. Now I have an internal PS/2 port.

HOWEVER.
The instruments change themselves on their own every now and then. Did I wire something badly or is the keyboard failing?

Yeah, I think would be my main problem..
If i had an instrument with a low envelope, I wasn't able to play any notes after that..

I'm gonna play with it some more tomorrow

Exactly what kind of problems are you having? Make sure you have an instrument in each note.

Ha, I just made an internal PS/2 port yesterday, after seeing your old post

It does work, but I still have some troubles with switching instruments and playing notes.
But it's still fun!

and it's easier to figure out the notes for a melody, because you can easily play it, without having to track it first..

Hey, yeah I saw your tutorial and it was helpful but I didn't want to steal your image without your permission. Thanks for contributing!

i made an updated image of the keyboard layout...

I am writing this to hopefully alleviate some of the frustration I had for other people. It all revolves around this one simple diagram I made:

I tried to make it as simple as possible. The pins on the gameboy are show as if you have just taken the front half of it off and it's laying in the position you would hold it in to play.

For the wire, you're on your own. Ribbon cable from an IDE cable is nice and flat and can easily be seperated into a strand with 4, but the it has multiple strands of wire so it can get a little messy. Also, you can strip this easily using your front teeth (I know, it sounds crazy but it works and I learned it from Ben Heck so it MUST be the best way). Personally I ended up using some copper wire that was probably way too thick for the job but oh well.

For the PS/2 ports, buy them online. Many people will tell you to get them off eBay, but those are drop shippers that ship from China and it takes almost a month (sometimes longer) for them to arrive. I went to pricewatch.com and got these:
http://www.sfcable.com/30D5-2B.html
I ordered two of them, just in case I messed up or wanted to do one on one of my other DMG's and with shipping it came out to about $5.00. I realized after that they had these:
http://www.sfcable.com/10M2-A3.html
So get one of those. It should be about the same amount.
You can try to buy some locally, but I went around to pretty much every store within 20 miles of my house and all I found were radioshack and Best Buy. They both wanted 20+ dollars for theirs, so I said screw that. I tried to get Best Buy to price match theirs because all it was was a generic one put in a dynex box. They wouldn't. You should never buy anything from Best Buy. The most help I got was a guy at Target that told me about pricewatch. Who knew!

To remove the plastic casing on the cables, just use your ingenuity. I used a combo of a utility knife (cut away from you!) and some pliers. It's not that hard.

To make the hole in the DMG, I used a dremel. A drill would probably work too, but you have more freedom to shape the hole as you want with a dremel. If you're going to be doing anything like this somewhat frequently they're a great investment.

As for the placement, like I said in the picture that is the easiest place. Just move the voltage regulator to the other side. I know that is a favorite place to put a pro sound mod, but I think that a 1/8 inch jack would be smaller and easier to figure out somewhere else to put it. I secured my jack in with some hot glue when I had it just right.

Now on to setting up the LSDJ file. Set it up to look like this:

Note that the tempo is cranked all the way up and that sync is set to keyboard.

Just hit A.

Hit A again.

Make sure you have all of the notes with an instrument. Don't actually put any notes in though.

Now go back to the song screen, hit start and you're ready to go!

For reference, here is an updated keyboard layout map from xero:

Also, here are the other commands from the manual:

I hope that someone will find this tutorial useful. If there are any questions ask away and I will do my best to answer them.

Special thanks to JustinThursday for putting up with my incredibly stupid emails, Rolf for the extremely helpful diagrams, and Nex and BetaSynapse for other tips.