It sounds like the Content Libraries where not installed when Revit 2015 was installed. They can be installed from the DVD/USB it was installed from. If you don't have that, or can't get access to it, students get access to Revit for free here:
http://www.autodesk.com/education/free-software/revit. You can install the whole software, or try to install just the Content Libraries. Of ourse, you'll need permission to install software on the computer, so you might have to ask the network administrator for help.
Also, for future reference, you'll get a lot quicker responses on your posts if you put a good description in the Subject. Just saying Revit 2015 doesn't tell anyone what the problem is. A better subject would have been, "Where can I find Missing Revit 2015 Content Library"
Anthony Tiefenbach
BIM Manager
HuntonBrady Architects