Visual Studio Extensions are a feature of the RadControls for ASP.NET AJAX installer. The best way to have them enabled is (as you've already tried) is to re-run the installer.
The message you report is a strange one, as running the installer with the products installed should just offer the Uninstall
screen. I'd suggest you the following steps to ensure that everything is fine:
- Start Control Panel -> Add/Remove programs
- Search for the RadControls for Visual Studio Extensions and remove them if found
- Find the previous installation of Telerik RadControls for ASP.NET AJAX and uninstall them
- Run the RadControls for ASP.NET AJAX installer again
- Select Custom in the Setup Type screen
- Ensure that the Visual Studio Extensions for Visual Studio 2008 feature is selected in the Feature selection screen
- Proceed with the installation
Let me know if any problem arises. I will try helping immediately.
the Telerik team