Do you mean SimulationXpress, then it only works on part. If
you talking about SolidWorks Simulation, then yes that can be used
for both parts and assemblies.
What version of SolidWorks you have. You need to have Premium
version of SolidWorks or need to have a license file for using
SolidWorks Simulation. Check in Tools > Add In if SolidWorks
Simulation is listed there.