Once you add that, you should be able to download and install the software normally using the Synaptic Package Manager.
However, there will be times when you can't find the software in a repository. If you're a newbie like me, my recommendation at that point would be not to bother. Find an alternative from the plenty of other Linux applications available.
Add/Remove offers only the most popular software, not the Synaptic Package Manager's wider selection. And you won't be able to add new repositories to it. But if you want to try it, here's now to proceed:
Select Applications --> Add/Remove, and the Add/Remove Applications screen appears. You'll find a list of categories, including Accessories, Education, Games, Graphics, Internet, Office and so on. Select the category you're interested in. You'll get a list of applications, descriptions and popularity ratings, gauged by the number of people who have downloaded it. Check the box next to any you want to install. Some of the applications may already have checks next to them -- that means that you already have the application installed.
Click Apply Changes. A notification screen appears asking if you want to "apply the change." Yes, the terminology is awkward, weird and confusing, but get over it --- this is Linux, after all.
Click Apply, and enter your system administrator password to continue. After minutes, depending on the size of the download, you'll get a notification that a new application has been installed. You'll also see the name of the program you just installed. Double-click it to run the application.
To uninstall the application, select Applications --> Add/Remove. To quickly find the application you want to remove, select Installed Applications Only from the top of the screen. You'll see a list with only the applications you've installed. Uncheck the box next to it, and follow the same directions for installing an application, and the application will be uninstalled for you.
The Update Manager
The Update Manager is best used for updating your system software, and any applications that came with your system, rather than any software you've subsequently downloaded. For that reason -- and others noted below -- I've found it confusing to use, although not all Linux users have the same issues with it that I have.