To attach a model, it should be the same as attaching a footprint:
In the SCH Library panel, double-click on your part. Click "Add..." button on the bottom. Select Simulation.
After that, never done it, but the tutorials from older Altium versions should guide toward the right direction.
I personnaly never used Altium for spice simulations. I much prefer LTSpice... maybe because of my masochistic tendencies.