If you want to play real media files in Debian Lenny, you may not be able to play it. If that is the case or you want to install RealPlayer anyways, just follow the simple instructions.
Download deb from http://www.real.com/linux/. Make sure you click on DEB Package instead of the Download Realplayer button.
Now, open up the terminal and type in su followed by your root password. Now type in
aup-get install lsb
Now, change the directory to the one where you have downloaded the DEB file. In my case it is the Desktop, so I changed the directory to desktop by typing in
Now, if you are on 32-bit version, and the name of the DEB file is RealPlayer11GOLD.deb you should type in the following:
dpkg -i RealPlayer11GOLD.deb
and if you are on 64-bit version (amd64), you should type in
dpkg -i --force-arctecture RealPlayer11GOLD.deb
Now, run it from the Applications > Sound and Video > RealPlayer 11 and then configure it.