I've been trying to install Adsense ads on a Simple Machines Forum with little success. Has anybody done this before? What was your method? Especially, where in the code did you put your Adsense javascript?

You should add the SMF AdMod. You can down load it from the smf website. If you want to hard code it into the template you can do that too but modifying it later can become a pain. http://japanlinked.com/forum/ this is my SMF forum if you want to see.