I would respectfully disagree with casper and recommend that you NOT install the Envy driver. It has caused myself and others a number of problems in the past. However, for future reference you can install .DEB files using this command in Ubuntu or other Debian-based distributions (replacing "nameOfYourDebPackage" with the actual file name):
I also tried downloading envyng from their website. The file that I downloaded opens Package installer but the "install package" button is greyed out, and I can't click it. Can I open the file that I downloaded in a terminal? If so what would be the command to run it? I tried just going to the download folder (in a terminal) and typing envyng-core_1.1.1ubuntu10_all.deb, but it's obviously not working.