Since Quartus II 10.0, ModelSim is directly available from the main installer. Please look at Install Quartus II

Download and install ModelSim-Altera

Altera provides a distribution of ModelSim software called ModelSim-Altera. It contains some additional libraries used to simulate Altera's components and it's very useful!

You can download it at the Altera's web site. You will find it in the same page you downloaded the Quartus II Web-Edition software: Quartus II Web Edition download page

Installation and licensing procedures are the same as Quartus. For more information take a look at the Quick_Start