When I ran ./configure --enable-gui I got the following message:
Checking for GTK+ version... GTK-2 devel packages were not found, trying GTK 1.2
Where can I get the GTK-2 devel packages?

Thank you.

I'd start my search at GTK+ (http://www.gtk.org).

You can find the GTK devel packages at Free downloads encyclopedia - Softpedia (http://www.softpedia.com).
Also you can download gnome-mplayer at the same site which have an easier gui.you install mplayer without using the option (--enable gui) and then install gnome-mplayer.
Reply me about the extent I helped you to solve the problem.