Which file did you download? There are two, there is a tar file, this is source, and needs compiling to make the executable binary file. The other option is a .rpm file. rps files are compiled, and only need installing via your package manager.
As for root commands, once you have the file, mplayerplug-in.so , you copy that file to the appropriate directory using the 'cp' ( copy ) command. You need root privileges if the directory is not in /home/username part of your file system.
Do you know how to install an rpm file? ( easy option )
Do you know how to compile a tar ball? ( more difficult option )