I had the same issue. Apparently in the latest version they don't define the Command parameter. So I just replaced Command with the literal command 'Delete' and that worked like such:
... Text="Delete" Command="Delete" Action="JavaScript" JavaScript="if (confirm('Do you really want to delete this event?')) dps1.eventMenuClickCallBack(e, 'Delete');"
Comment posted by Dan Letecky 15 years ago.
This is fixed now in the sandbox (since build 6.1.2121).
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