the software in the software portal can be from different sources/servers/sites than adept uses so adept will not list those software packages.
using the portal and mintinstall allows you to install software from sources/servers not used by adept (or synaptic).
adept is for KDE and synaptic is for Gnome.
multimediasupport.mint is only for people who have installed the light version of mint and there is not a light version of Cassandra KDE.
if you want to install western-quake3 why not run the mint file instead of saving it?
so if you have saved the mint file open a terminal window (konsole) and type:
sudo mintinstall ./western-quake3.mint
that is if you saved the file to your home directory.