Not sure about playmaker, but I'm guessing it doesn't use a multi-line input field. NGUI does, so when you press enter when editing the label's text, it creates a new line. Check with that plugin's author.
If you type \n in inspector, it's two characters. The \n is only processed when done via code, which is why I mentioned. In inspector on NGUI's side, you just press enter. Playmaker though? No idea...