I would suggest you to do the following steps in Visual Studio 2010:
1. Open the Extension Manager.
2. Select Installed Extensions tab on the left and check if Telerik WPF VSExtensions are currently installed. If they are not installed select Online Gallery tab, type Telerik in the search box, select Telerik WPF VSExtensions and click on the Download button. After the installation is complete restart Visual Studio.
In case Telerik WPF VSExtensions are available in the Installed Extensions tab I would advise to perform the following steps:
1. Select Telerik WPF VSExtensions and click on the Uninstall button.
2. Restart Visual Studio.
3. Open the Extension Manager and select the Online Gallery tab, type Telerik in the search box, select Telerik WPF VSExtensions and click on the Download button.
4. After the installation is complete restart Visual Studio.