Sooo, you're trying to plug a 2-wire fan into a 3-pin mainboard header, or similar?
The third wire, if you didn't already know, is the RPM sensor.
You can simply plug the 2 wires you already have into their respective pins, and link either the ground or the live to the third pin.
I forget which one it is, but I would obviously recommend trying the ground first, as thats not going to damage anything, whereas if you try the live first and it's wrong, it probably wouldn't be too good.
If you have a 3-wire fan, and a 2-pin header, just ignore the green sensor wire.
In any case, unless you have 3-wires and 3-pins, you're not going to get the RPM sensor working...