You can download Midi2CV Mk2, MidiARP and MidiSEQ source code using the following links:

Midi2CV Mk2: midi2cv_mk2_112_src
MidiARP: midiarp_095a_src
MidiSEQ: midiseq_097a_src

The code can be built using WinAVR-20100110 toolchain available here: https://sourceforge.net/projects/winavr/files/WinAVR/20100110/

Have fun hacking!