We are aware of the incomplete Blend support of the RadFluidContentControl and we worked to improve it for our 2010 Q1 release, which will be made available by the end of this week. However, due to the nature of the control, it may still be tricky to edit the three content properties. Because the logic of the control dictates that it changes its visible content according to the space that it can occupy, if you simply drag and drop the control on the design surface you will only see one of the contents. So, to be able to edit all content properties in Blend with the latest version of the control, you need to set the ContentChangeMode to "Manual" and then change the State property in the property grid on the right to the content that you want to edit - Small, Normal, or Large. After finishing with the editing, depeneding on your specific application logic, you may need to return the ContentChangeMode property back to "Automatic".
Please try editing the control in Blend once we release the 2010 Q1 version and let me know if you have any difficulties.
the Telerik team
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