I recently installed an official .ova for Kali, in VirtualBox. When I try to add Plasma via APT, it has never heard of it. I assume this is due to not having the correct repository. What repo should I add in order to get Plasma from APT?

thanks

What do you mean specifically that it has 'never heard of it' ?

Probably just need to install the correct packages, something like this
https://www.kali.org/docs/introduction/ ... ironments/

I don't think Kali uses separate repos for things like this.