The installer for forge should be run from some temporary place, not any place you plan on having the final system installed to.. Though, it should also be putting out files with a different name than the installer, so technically it "shouldn't" matter.. but..
So, have you actually logged into your 1.6.4 -world-? For the installer to work, you have to have launched the 1.6.4 profile at least once.. past just logging in, and into the screen where you can pick a single player world to run. That initial running is what downloads all the game pieces for that version, and Forge needs those pieces already there to install.
And I've found that the .exe installers don't always work nice, the .jar is the way to go... Also, if you already have a forge installed (for the 1.6.4), it might not try to install again.. have you tried creating a new profile and seeing if you have access to a Forge for 1.6.4 as a profile/game?