Originally Posted by ShneekeyTheLost
Yea... you need to delete META-INF before starting to install mods. Deleting it after you install Forge does Bad Things as well.
Start with a fresh install. Delete META-INF, drag and drop in Forge. Run it. See if that works.
Also make sure you have the latest edition of Forge that comes with FML and the 4096 fix.
DO NOT use Modloader. It's not necessary anymore.
I'm switching to MultiMC again. I ran it vanilla, then ran it with only forge, then tried to run it with everything... but despite all that, it still said "Error: the directory is not empty."