Paste into Xterm on MacBook Air
If you're using an external mouse, clicking both buttons (if it's a two button mouse) should do it. 

I'm not aware of a way to do it with the trackpad only unless you use a third party tool like KeyRemap4Macbook which will let you bind, eg, ⌥ + left click to a middle click. I've just tested exactly that combination and it works fine for pasting into an X11.app Xterm so should work fine with your VM.