On Ubuntu (at least from version 7) it's as easy as this:
- open the Applications menu from Gnome (or equivalent for KDE)
- open the Add/Remove tool, at the bottom of the menu
- select to search from all possible reposities, not just the open source/"free" ones that are the default
- make sure the category "Everything" (or equivalent) is selected from the left pane
- type 'rox' or some other search word to the search box and start the search
- if the app was found, select it from the list to be installed
- if it wasn't found, it's not in the official Ubuntu reposities and you'll need to hunt the .deb files from the web or add a reposity that holds it (note: adding extra reposities is at your own risk)
EDIT: with apt-get I'd just alter the reposity files to enable all reposities (that suit the situation), then
Code:
apt-get update && apt-get install rox