Yes, you haven't add the reference to CuteEditor.dll when you are using the CuteEditor in the code-behind mode.
As the error message suggested, you need to add a reference to CuteEditor.dll.
Steps:
1. Right click on the project and select "Add reference".
2. Click the browse in the new window
3. Browse to the location of CuteEditor.dll and clcik the open.
4. Click OK.
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