A google search for "maemo extras-devel mplayer" gets me this in the second result.

Another search for "mplayer n810" finds this in the first result.

Yet another search for "mplayer os2008" gets me this , also in the first result.

A search on these forums of all three of the above queries gets me another set of useful threads which almost all mention the extras-devel repository (several of them specifically how to to install)

A search of http://www.gronmayer.com/it would've pointed you directly to the repository you needed to install, too.

Call it whatever the hell you want, but I'm just trying to point you in the right direction, and certainly gave you enough info to get you started if you had been willing to put forth a little effort.