All credit for this thread should go to pomeroyip, as he set up the original thread on the old Custom PC forum - I'm simply recreating it here for continuity.
As this question gets asked so often, here is how to do it:
To add your stats to your sig enter the params surrounded by the [folding] tag into your forum profile sig box.
Example:
[folding]u=FAHUSERNUMBER&t=TEAMID[/folding]
You can get your folding username from the end of the URL on
http://folding.extremeoverclocking.com
i.e.
http://folding.extremeoverclocking.c...php?s=&u=83772