Eelco,
"holder" is not supported in the current version.
Here is the syntax to add new button or dialog:
editor.
CustomAddons = "<img title=""Underline"" class=""CuteEditorButton"" onclick=""ShowMyDialog(obj_Editor1)"" type=""btn"" src='CuteEditor_Files/images/emrainbow.gif' hspace=2 border=0 />"
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