Please make sure that items in your toolbox are from the same assembly version as the project references.
As for the other problem - could it be that the RadMenu1 is placed inside a templated control? This way its ID cannot be used directly, but first need to be found with FindControl.
Are you using a web site or a web application? If the latter - make sure that the .designer file contains the proper definition of the menu.
the Telerik team