How to add OMC to ID 1.6.8.md

Open IDE Press Ctrl + Comma Insert following link in additonal board manager URLs near the bottom (one url each line should you have any beforehand)

https://raw.githubusercontent.com/ThamesValleyReprapUserGroup/Beta-TVRRUG-Mendel90/master/Added-Documents/OMC/package_omc_index.json

A more updated "package" with support for more chips including the 644 and 644p along with 1284. it also has the benefits that you can select brownout etc from ide:

https://raw.githubusercontent.com/ThamesValleyReprapUserGroup/Beta-TVRRUG-Mendel90/master/Added-Documents/OMC/package_MCUdude_MightyCore_index.json

HINT: pres the button at the end of the box to open a small editor and insert link there

after that go menu tools >>>> Board >>>> pick board manager at the top... in the next selection box OMC should be at bottom of the list... click install