"The design unit was not found" error in Modelsim

it always goes like this. when you open modelsim-altera's modelsim, you see all the libraries compiled and ready. but your simulation will never see them. don't know why. i usually manually compile few libraries for my project in my newly created library. this way the simulation recognises the new libraries. my problem is that the simulation can not recognize my variables, register names, pins, so on. as if they were unknown libraries. thats what bothers me.