SC3plugins
should already be on the board, and ATK should be part of it.
Instructions to build SC3plugins on Bela are here, however they are fairly outdated. Probably something like this would work today, after you copied this supercollider source to /root/supercollider
and the sc3-plugins
source to /root/sc3-plugins
cd /root/sc3-plugins
mkdir build
cd build
cmake -DSC_PATH=/root/supercollider -DCMAKE_C_FLAGS="-march=armv7-a -mtune=cortex-a8 -mfloat-abi=hard -mfpu=neon -O2" -DCMAKE_CPP_FLAGS="-march=armv7-a -mtune=cortex-a8 -mfloat-abi=hard -mfpu=neon -O2" ..
make
make install
but - again - they are already on the board so you shouldn't need to do the above.