ModelSim - Design unit not found

I'm trying to use ModelSim to simulate a Verilog design that requires the megafunction lpm_mult. I have found entries for lpm_mult in the 220model.ver and the lpm.ver libraries. In the Start Simulation window I have tried clicking on (highlighting) one of these versions of lpm_mult in addition to my own project files, but I always get the error message:

** Error: (vsim-3033) C:/Documents and Settings ... qmul_alpha1.v(57): Instantiation of 'lpm_mult' failed. The design unit was not found.

qmul_alpha1.v is the file containing the reference to lpm_mult that was created by the Megafunction Wizard.

What do I need to do to correct this?