well...Mint is Ubuntu, so RPM files only with alien. But, once you new to Mint my guess is that you just are trying to run rpm in a debiam based system. RPM equivalentes are .deb files here
anyway, you can install this software like this:
- open your terminal and type:
wget -q http://www.ap-i.net/pub/patrick_chevalley.gpg
-O- | sudo apt-key add
- now edit /etc/apt/sources.list and add this line at the end
- now update
- no search for the packages skychart and skychart-pictures in synaptic.