You probably have Java 8. 1.6.x and 1.7.2 versions of forge won't work with Java 8. 1.7.10 and up will work, 1.5.2 and below too. I suggest you to download this small thing called Java legacy fixer and put it into your "Mods" Folder. Then you will be able to play 1.6.4 MC forge.
Just tried to install a few mods for 1.6.4, clicked "Play" and nothing pops up. Help?
A log would help
You probably have Java 8. 1.6.x and 1.7.2 versions of forge won't work with Java 8. 1.7.10 and up will work, 1.5.2 and below too. I suggest you to download this small thing called Java legacy fixer and put it into your "Mods" Folder. Then you will be able to play 1.6.4 MC forge.
http://files.minecraftforge.net/LegacyJavaFixer/legacyjavafixer-1.0.jar
Stuff I support