No Plug-in tab on Administrator panel???

Why can’t I find anything for Plug-In’s? On my 2nd WordPress site that I’ve just been asked to help manage, it is a tab on the left, after Appearance. But on my primary site it is not there. I cannot find anywhere to add Plug-ins on my Dashboard either. Is there a plug-in to add plug-ins? Would like to add an event-calendar and have found some great plug-ins, but can’t add them!!