Please visit NAMD website. It does contain the exact tutorial you are looking for.

http://www.ks.uiuc.edu/Training/Tutorials/

Here is the exact link, if you using linux/Mac: http://www.ks.uiuc.edu/Training/Tutorials/namd/namd-tutorial-unix-html/node16.html

and the required files: http://www.ks.uiuc.edu/Training/Tutorials/namd/namd-tutorial-files/

I suggest you also try constant velocity pulling as it will give you a force-extension curve, which is often more informative than constant force pulling. 

Best,

Boon