Arduino board definition
The JSON file at https://raw.githubusercontent.com/getopenmono/arduino_comp/master/package_openmono_index.json points towards files hosted at localhost ... this doesn't work :)
The file is the one referenced in your documentation on how to use the Arduino IDE with Mono.
@mhvelplund , thanks for your heads up. I am working on finishing the board package and API's. See the Announcements category.