Hi Brett Parkhurst,
There is such a problem, that VS cannot resolve correctly the path to any of our assemblies– we have reported this issue to MS and we are working closely with them to resolve it as soon as possible. As a workaround you can manually copy all our assemblies to the VS public assemblies folder and everything will work as expected. In fact we have the same problem with our Silverlight 3 assemblies, if you try to add them manually to the VS2010 toolbox you will get the same error. However, our installation deals with it by some registry entries.
However there is also another problem that you need to know, if you add our SL4 assemblies to the toolbox and you have our SL3 assemblies installed you won't be able to drag any of our SL4 controls from the toolbox due to an exception. Therefore, to make our SL4 assemblies add to the toolbox and work properly you will have to perform the following steps:
1. Uninstall Telerik SL3 controls
2. Manually copy our SL4 assemblies to the VS Public Assemblies folder and add them manually to the toolbox
We apologize for this inconvenience, this is why we posted that we don't have VS2010 toolbox support in our SL4 related blogpost
. But still this is only a beta for both VS2010 and SL4 and I can assure you that we are working closely with Microsoft to resolve those issues. If you think that these workarounds are too much trouble you can still try our controls without toolbox by just writing XAML.
Hope that this is helpful. If you have more questions please let me know.
the Telerik team