OK, I'll try to explain things a bit....
When it says "available now", that means it's available to go on the "queue". When people request a track, it goes to the back of that. You can see the queue
here. It's like you're getting in line for the ride. You can get in line now, but you still (unfortunately) have to wait for everyone in line in front of you - line jumping is BAD. Once a track goes on the queue, it stays there (even if you log out).
So for the three things a track might say there:
A green "Now" means the track can be put on the queue right now (and played when the tracks before it are finished). A gray "Now" means the same, but you don't have enough points left to request it.
A white "In Queue" means that the track is already in the queue somewhere (and therefore, it's pointless to request it anyways)
A red date and time means the track was requested and played recently. To avoid the same tracks repeating over and over, you can't request a track within a certain time of it being played (I think it's, what, a week in between requests). The date and time is how long it will be before the track can be requested again; at that date and time, the track becomes a green "Now".
I hope this explained it kinda OK ... or there's also the
FAQ to help if you need it.