*.package
 

how do I install a *.package file?

Not entirely sure what kind of file that is. Here it implies it's related to the SIMS game. Is that what you're trying to install?

Sorry, I just found AutoPackage. Maybe it's more what you're looking for (at least it seems it uses *.package)...

it's a program I found over the net : celestia, related to gtk

and where did you get the file? here is where you can get an rpm for fedora of celestia
http://dag.wieers.com/packages/celestia/

Add dag to your repos if you are using apt and apt-get it