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Makey Makey
Follow the links in the above link.
Using Arduino IDE to program Makey Makey
Instructions on the Instructables site above are a bit vague. However, they work : unzip in the Arduino folder and restart the IDE. My indication of vagueness results from other experience of imprecise directions in adding functionality to the IDE.
makeymakey.com
With the above setup the compilation of test code proceeds OK but fails to upload due to absence of avrdude. 'Whereis avrdude' finds avrdude so try another installation.