Not sure what you mean , plug ins can come stock or bought from a third party company. Cubase comes with some samples, loops, one-shots, but those also can be found from 3rd party providers. Check out my section on channels many good tutorials there.
there is a folder in your cubase Directory especially for plug ins - there you put the plug ins in (bought or free) and cubase will automatically find them, if not then you go to the plug in menu in cubase and manually load them from your harddisc (enter hard disc directory). thats at least how i did it...
