No. You want package B unless you have a really old processor. And don't forget the other programs mplayer is dependant on, like Lame. They used to list these on the mplayer page, but they're gone now. You'll get the list when you try to install mplayer. All of the necessary files are available at packman.
The reason you have to type in the complete path for the program is because, when you click other, it opens up a window with your home directory as default. At least that's what my machine does. And the programs you want to use are usually in /opt. If you change to that directory, I think the command with %U symbol will work.
