An IDE is just a shell around the command line. I had a quick peek at the instructions and see no reason it will not work. I installed it, but it does not show up. My other entry in 'hardware' does, it's the bare atmega328 IIRC.
Here are screen grabs of my 'hardware' folder, it now has 2 extra boards. Let me know if something needs to be changed in order for the 'mighty-1284p' stuff to show up.
Hi @interfan2019. It is impossible to use that boards platform with any version of Arduino IDE higher than 1.0.x:
Even though the changes to make it compatible with newer versions of Arduino IDE are trivial, the creator of the platform had already abandoned the project long before then so it never happened.
The platform is of historical value as one of the pioneering efforts by the community to expand the Arduino hardware ecosystem, but that is its only value.
Use the superior and reasonably maintained MightyCore as recommended already by @westfw. This is the platform used by the great majority of the Arduino community. It can be installed very easily in Arduino IDE 2.x and works perfectly.