Problem with Quartus II (Web edition) RTL simulation with Modelsim (student edition)

If you effectively installed it, the path is easy to find:

assuming you are using windows

click Start

click All Programs

scroll to the folder named ModelSim or similar, click on it

then right-click on the Modelsim executable and select 'Open File Location' from the pop-up menu, a WIndows Explorer Window will open.

Right-click in the address-filed of that window explorer window and select Copy Address from the pop-up.

Paste that in the appropriate place in Quartus' Assignments->Settings->Eda Tool Settings.

I gather somebody else will help you with Linux, if that is what you are using.