In the Previewer, there's a model called Nullfield.m3, located at Assets->Effects->Protoss
How can I get an ability or effect to use this, so when I hit the ability button and place it on the ground, it appears? In this case, I'm trying to replace the mothership's vortex graphic with Nullfield.

Cheers

If you are just trying to change the motherships vortex then change the vortex model actor's model to be a model that uses the Nullfield.m3. Create a model if you need to, set it's parent as OneShotSpellFX (type should be generic) and change the Model property to be the Nullfield.m3.