Using an opto-isolator in a car's circuit. How to ground?
Ignoring the question of whether you actually need an optoisolator for the moment, the specific problem you're having in the circuit simulator is that the simulator itself requires a DC path among all nodes of the circuit being simulated &mdash; otherwise, the math does not converge on a solution.

The traditional fix for this is to connect a high-value resistor (e.g., 10M&Omega; or more) between your two "isolated" grounds only for purposes of simulation. It will have negligible effect on the actual solution. You would do this wherever you have an "island" of circuitry that is totally isolated from the simulator's ground (node 0) by capacitors, transformers, relays, optical couplers, etc.