I'm not sure which microscope you have, but there should be a bit that pulls of, and the bit that goes inside should be flat - that is the bit you put the saliva (or sperm if that's what you're trying to see) on.
I've never seen sperm through mine, but I only tried it once, and that was months ago before reading this thread, I assumed they were too small to see. I believe sperm are the smallest cells in the human body, I tried looking at blood cells to get an idea of how small sperm might be, so I figured if you could see sperm cells, then they would be like tiny dots.
Anyway, if the semen sample is dry I'm pretty sure the sperm would be dead and so you wouldn't see any movement which I expect would be the indication that it is actually sperms you are seeing.