equation system

I want to use the following bidomain equation in my simulation :

∇·((σi +σe)∇φe)=−∇·(σi∇Vm)

I have two spheres one within the other as geometry. Inside the smaller sphere I want to introduce Vm as a point source and set it to 1 Volt and measure the potentials φe on the larger sphere's surface. I assume isotropy and give conductance values constant for now.

How can I enter this equation in COMSOL and do this simulation?

Thanks,
Gizem