http://reprap.org/mediawiki/images/6/62 … ard300.png
http://reprap.org/mediawiki/images/d/d5 … tic150.png
http://depts.washington.edu/open3dp/wor … _image.jpg
Rev E has the same 4-pin power connection as rev B-D. Rev F increased this to a 6-pin version.
-ve goes to the 2 pins closest to the edge of the board.
+ve goes to the other 2 inboard pins.
This should be how a normal ATX 4-pin connector is wired up, and can only be inserted one way, so yellow = +12V, black = -ve as jago says.
If you're hacking up the existing supply, or using a new one, I highly highly recommend confirming you check the polarity is right with a multimeter before you plug it together!! Getting this wrong will cause board death. 
SD3. Mk2b + glass, heated enclosure, GT2 belts, direct drive y shaft, linear bearings, bowden-feed E3D v5 w/ 0.9° stepper
Smoothieboard via Octoprint on RPi