I also had this issue. I tried many things with test projects in 3.5 .Net. Non seemed to work. I was successful in using RadControls in 3.5 projects, but not Prometheus. I test various different things over the course of 4 hours, including full uninstalls of both Q2 and Q3 Prometheus.
I finally hit on the answer, and it has to do with the GAC. There are two versions of the Telerik.Web.Design installed in GAC, one appears to be for 2.0 and the other for 3.5. The 2.0 seemed to work correctly in my test projects. However the 3.5 designer would never load. It appears that the 3.5 designer cannot find the Telerik.Web.UI.dll. If however you installed Telerik.Web.UI into the GAC, then all is better.
Procedure to do that is:
- Use the VS 2008 Command Prompt. If running Vista, right click the icon and run as Administrator. This will get the directory for the gacutil program into the path statement.
- gacutil /i "C:\Program Files\Telerik\RadControls 'Prometheus' for ASPNET Q3 2007\Bin35\Telerik.Web.UI.dll"
You should get a confirmation that it is installed.
This was not previously required in Q2, having Telerik.Web.UI in GAC. Not sure why it is now.
You can list out GAC with gacutil /l > c:\temp\gacoutput.txt
Then you can review what was installed. With both Q2 and Q3 installed, it should look like this (after the above fix):
Telerik.Charting, Version=18.104.22.168, Culture=neutral, PublicKeyToken=d14f3dcc8e3e8763, processorArchitecture=MSIL
Telerik.Charting, Version=22.214.171.124, Culture=neutral, PublicKeyToken=d14f3dcc8e3e8763, processorArchitecture=MSIL
Telerik.Web.Design, Version=2007.2.1107.0, Culture=neutral, PublicKeyToken=121fae78165ba3d4, processorArchitecture=MSIL
Telerik.Web.Design, Version=2007.3.1218.20, Culture=neutral, PublicKeyToken=121fae78165ba3d4, processorArchitecture=MSIL
Telerik.Web.Design, Version=2007.3.1218.35, Culture=neutral, PublicKeyToken=121fae78165ba3d4, processorArchitecture=MSIL
Telerik.Web.UI, Version=2007.3.1218.35, Culture=neutral, PublicKeyToken=121fae78165ba3d4, processorArchitecture=MSIL
It is unfortunate that it took so long to figure out. By the way this is with Visual Studio 2008 running on Vista (x86).
Hope this helps, Mike