The 'su -l' command is bringing you to root's home directory, so make sure to go back into your /home/kostis/packages directory.
Also, if you aren't using tab completition try it out. Once you enter into the correct directory do "rpm -ivh MPl<press tab>" Pressing tabs tries to fill in the rest of the file for you.
The entire thing should look like:
Code:
kostis@linux:~/packages> su -l
Password:
linux:~ # cd /home/kostis/packages
linux:~ # rpm -ivh MPlayer-1.0pre8-8.pm.svn20060811.i586.rpm