Have you tried starting Windows Explorer as Administrator?
Even if your user account has admin rights you still need that elevation for such tasks.
If this doesn't work, I suggest you use gacutil.exe. You also have to open the command prompt as administrator then:
- In start menu find the link to the command prompt of your visual studio installation. If this isn't your development machine you need the SDK or just copy gacutil.exe.
- Right click the command prompt shortcut and choose "run as administrator".
- In the command prompt you type gacutil /i "assembly_path". Or type gacutil /? for an explanation how to use it.