Only in the windows or linux world is it reasonable to expect the user to hunt down all the required libraries to do a simulation. Which is to say it is reasonable, I guess, in this unfair world.
Anyhow, when doing a ModelSim on my Actel ProAsic3e design, my module used a primitive, which consequently required the proasic3e library to simulate. To get ModelSim to know where the library was, it required two nonintuitive steps. First, I had to use the vmap command on the ModelSim console to map the name "proasic3e" to the physical library file. This also required that I figure out where it was hidden on my system, which I would never have been able to do without hints from various forum threads. Secondly, I had to specify the path name with forward rather than windows (backwards) slashes. The resulting vmap command is:
vmap proasic3e C:/Actel/Libero_v8.6/Designer/lib/modelsim/precompiled/vhdl/proasic3e
Now, "proasic3e" appears on the library tab and everything compiles and works.