As a little background, Linux distros handle program (package, in Linux) installation in a far different way from DOS/Windows.
Most distros have "repositories" ("repos") in which packages formatted for installation in that distro's format are stored. When you want to install something, you open your package manager (software center on some distros), search for the appropriate package, then tell the package manager to install it. In the process, the package manager resolves "dependencies," that is, any libraries or other programs that the target program needs to run.
If that doesn't work, you compile software from sources, which usually is far less complicated than it sounds.
gives a pretty good overview of the process.
Since the Debian DVD is in your sources.list, you can use your package manager to install software from it, though it would be easier to remark it out and install programs from the repos, as long as you have an active internet connection.