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I am new to linux but go into syanptic and either d a search or else manually search for what you are looking for - the is the restricted stuff, universe and multiverse areas for the various application types in the list.
there is obviously a command line way too but I am afraid I cannot help you there.
if the repository is not enabled, that may be why you cannot get it. It should be under an options menu.
If it's there and not checked, heck it, or if it's not there at all, you could add it by just hitting add, and fillingin the blanks!
(do a google for all the details..just google ubuntu universe repository, and it'ss point you towards some of the mirrors.