To show assembly from the GAC in the Add Reference dialog box

When you add an assembly in the GAC(c:\windows\assembly), it does not show in the add reference dialog box automatically, instead you have to make it visible in the add reference dialog box by updating the system registery.

Following are the steps:
1. Run the regedit.exe
2. Registry Editor will open, now traverse to the following location
[HKEY_CURRENT_USER\SOFTWARE\Microsoft\.NetFramework\AssemblyFolders]
3.Now right click on AssemblyFolders and Create a new key with the name of the folder from where you have added the assembly into the GAC (make sure that assembly should be there).
4. Now right click on the default value under the newly created key, choose modify option a dialog box will popup, enter the path of the assembly folder in the value data and click on ok.
5. Close the registry editor and restart the VS.Net.

If you want this setting for all the users of this machine then follow the above steps under [HKEY_LOCAL_MACHINE]