ModCoupler-Verilog in version PSIM 2020a is compatible with all versions of ModelSim (64-bit only). ModCoupler-VHDL in version PSIM 2020a is compatible with Questa Core 10.0 (64-bit only).
Modcoupler-VHDL needs the FLI (foreign language interface) feature to communicate with ModelSim. Nowadays, FLI is a pay-only feature, available only in Questa Core.
Modcoupler-VHDL does not work with ModelSim-Altera nor any other free Modelsim variant.
Modcoupler-Verilog works with any Modelsim or QuestaSim version. The required feature (VPI) is part of the Verilog standard, so any simulator must support it.