Back of the forarm? If your arm are out at your sides, and palm is facingdown, would it be the top of your arm? That would be the brachioradialis. To stretch you need to lock out your elbow at your side, make a fist like you are holding a hammer, and flex your lower forarm hard.
You can rotate your fist to hit different areas. The strecged muscle is on top of the arm in the photo.
http://img234.echo.cx/img234/4453/me0022ij.jpg
I hope this helps