Did you copy CuteEditor.dll to your project bin folder?
Have you added reference to CuteEditor.dll in the top of your page?
For example:
<%@ Register TagPrefix="CE" Namespace="CuteEditor" Assembly="CuteEditor" %>
asp.net Chat http://cutesoft.net/ASP.NET+Chat/default.aspx
Web Messenger: http://cutesoft.net/Web-Messenger/default.aspx
asp.net wysiwyg editor: http://cutesoft.net/ASP.NET+WYSIWYG+Editor/default.aspx
asp wysiwyg html editor: http://cutesoft.net/ASP
asp.net Image Gallery: http://cutesoft.net/ASP.NET+Image+Gallery/default.aspx
Live Support: http://cutesoft.net/live-support/default.aspx