you will first need to put the archive into the folder where you want to run the game from then do the following command "tar -zxvf minetestc55_20110731-3~natty1.tar.gz" without the quotes.
That will extract the archive into the current folder, then you just need to cd into the bin directory and then do the following command "./minetest" to run the game ^_^
I'm not 100% sure where you got that package from as I play minetest on windows, but as long as its been built the usual way then what I've said above should work, if however its been built differently then the minetest file may be in another dir.
But any problems give us a shout mate.
EDIT: ok found where you got the package from, that is actually the source code that you have ^_^ you will need to download one of the builds
amd64 if you have a 64-bit system or
i386 if not :D then you run the command "dpkg -i file_name.deb" that Sfan5 posted.
Sorry for the confusion wasn't sure where you had got that archive from.