We migrated all the source code and setup files to a new Github repository. Detailed instructions are right there in the README file, which is automatically displayed at the bottom of the page. You can download source code, binaries, and all the setup files you need to get Arduino to compile for the ATmega644PA in your synth.
If you make some headway with the software, please apply to make a branch on the github repository and we will work on it together.