I think I ought to point out, after reading another thread by the OP, that from the command line packages can be installed by using the following commands:
apt-get install <packagename>
The update command only needs to be run once per session but is good practice to make sure that your software sources are up to date. The package name can usually be fond using hte Ubuntu Packages Search website:
The Software Center isn't required to using Ubuntu and installing software it is just a snazzy front-end to the actual package management tools.