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Have you first checked that the Office plug-in for publishing from MS Office has been properly installed, and that the Web site you are trying to publish to, has been activated? You need to activate each Web site you want to publish to.
Also, check that you have proper permissions to publish from Office. In EPiServer CMS 5 you need to set the permissions both in the Admin mode and in the web.config file, for the publishing to work. This is described in the EPiServer CMS 5 Administrators Manual.
Hope this helps.
Regrads, Andrea