Assuming you are in Synaptic it should look similar to this:http://blog.gambliser.com/wp-content/uploads/2012/02/linux-mint-12-4.jpeg
The box on the top right shows all packages that match your search criteria, both those already installed (if any) and those available but not installed (if any).
Those installed have the square at the left edge of the line in color (green I think). Those not yet installed are blank. Highlight one you'd like, right-click and select for installation (same thing can be done in the package menu at the top of the window), then apply (that green checkmark button at the top of the main window). It should go about installing the one you chose, plus any other packages it depends on to run.