If you've installed Modelsim Altera/Intel edition, then to enable the tool, you need to specify the installation path in the Quartus tool. To do so, do the following.
1. Launch Quartus tool
2. From the menu choose Tools --> Options.
3. In this Options window, look at the list in the left pane. Select "EDA Tool Options" from the list.
4. Now, in the right pane, you will see a list of tools with blank text boxes. Choose the box with [...] next to ModelSim.
5. Browse to the location of Modelsim installation and point to the Win32 folder.. for example ..
C:\intelFPGA\17.1\modelsim_ase\win32aloem .
6. Click the "Select Folder' button and close the windows.
Now, you can launch ModelSim from the Quartus menu for Simulations. Provided you have a valid license for it.