Did this problem occur after an upgrade from an older version of the RadEditor for MOSS? If yes, then you should read the upgrade instructions from http://www.telerik.com/help/radeditormoss/Upgrade%20to%20a%20newer%20version%20of%20RadEditor%20for%20MOSS.html. The problem will be fixed when you add the appropriate <SafeControl> and <dependantAssembly> tags for the old version into the web.config file.
If this is a new installation, then you should simply make sure that the version (188.8.131.52 in the code fragments you posted) matches the one you have installed (the latest version is 184.108.40.206).
If you are sure that the two versions match, then I suggest that you check the GAC of the MOSS server and verify that the RadEditorSharepoint assembly is there.
the Telerik team