I wanted to add Mekanism as a mod to the pack but it doesn't load.

This is a fabric pack, the mod you are trying to add is a forge mod.

Oh Wow i am stupid. I didn't even notice. well i guess i gotta find another modpack then.

https://github.com/BratishkaErik/Mekanism-Fabric
There is a Mekanism Fabric Port but it is in an early stage.