you download the file into any directory, then while in that working directory you type:
Code:
dpkg -i (packagename.deb)
Of course you don't use (). If the package in in in your sources.list you can:
Code:
apt-get install (packagename)
Notice that when you apt-get you do NOT type .deb. If you are still confused feel free to repost again, I'll keep an eye out for you.