If you're trying to get someone and can see a post of theirs, in the left side underneath their profile photo you'll see some small icons. One looks like a scroll, that's the PM button (if you hover the mouse cursor over each one it will tell you what it does). Click that and you should be sent right to a PM screen.
Or, if you click on their forum name, it will take you to their profile and under their photo you will see a link labled "Send PM" and you can click that.
Or, under "My Messages" you can directly link the send a message screen. You'll need to know their forum name but it should try to autofill for you. When you type it in, it will pop up the suggested name and you need to click that.
Hope that helps, not sure which part you needed help with.
-steve
