I thought you were supposed to enter every course you've ever taken, regardless of whether you got credit for it, failed, withdrew, re-took, or whatever. That's what I'm doing, at least. If you don't enter the W course and it shows up on a transcript, they might think you were trying to hide it from them and hold it against you in some way. Dunno for AADSAS, but I'm absolutely sure this is true for TMDSAS (the one for TX schools only).