When someone's profile says "online now" does that mean they are on umd? or that they are simply using their computer right now.
Im asking cause i want to chat with someone who says "online now" but when I try to chat it says "I'm offline"

It means they are on the UMD site, or have been active recently, but the user has options to disable chat and also to set a status, which is probably what you're seeing.