Hi there,
The most common items that you may want to back up can be added with the backup sets. Things like Mail's email, Address Book, iCal stuff, Keychains, bookmarks, etc. Anything else, you can either add the Library folder to the Finder sidebar, then in Advanced under add File/Folder you should get a standard system file chooser which shows the Finder sidebar and you can click on that to be taken into the Library folder, or you can open up what you want to backup in Finder and drag and drop it into the Advanced view under the Back up section.
Thank you,
Carl Cochran
Mozy L2 Support Engineer