1. You must be a member of the Administrator Group.
2. Navigate to "C:\Windows\assembly" in the explorer and right click on it and select "Open in new window".
3. Type 'cmd' in run and as it appears in the top, right click on it and select "Run as administrator".
4. In the command prompt navigate to "C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin\x64"
5. Now use the following command: gacutil -i "full path of the file system\XYZ.dll"
6. You will get a message "Assembly successfully added to the cache"
7. Refresh the assembly folder that you have opened in step 2 and you will find your applications assembly there.