Is there a way (or a plug-in) to see the IP of a contact?

You can know the IP of you contact if he/she tells you what it is, they can use (!IP) for this, or you can use netstat.

But what if he doesn't have messenger plus?

You can ask him. He can see his own IP by going to start --> run --> typing "cmd" (without the quotes) --> pressing ok --> typing "ipconfig" (without the quotes) in the DOS window and then he'll see his IP .

The problem is that i want to contact his FTP server, but he has a dynamic ip. When he is online he tells me what he's ip is, but when he has the status away, he can't tell it to me.

There are a few organisations that allow you to have a constant URL bound to a dynamic IP address. Two such organisations are DynDNS and TZO. If your contact gets an ftp:// URL from one of those, then you can access his server using that URL instead of an IP address.

cant he set an auto-away message saying what his IP address is.

Or use something like www.no-ip.com