I have checked my own set up in P8, and every time I enter the material room, the Add button does appear.  However, currently I am running on XP.  Is your P8 installed into Windows 7 Program Files folder or is it in it's own folder on the core drive?  If it is in Program Files, do you have all security authorizations in place?  And have you installed all the Service Releases? 
This is information you didn't provide in your OP, and it could conceivably be something blocking access or perhaps an upgraded not yet installed.