The reason that it is not enabled is because the document that you have as
the active document is open in compatibilty mode. If you save it as a Word
2007 document, or create a new document in Word 2007, it will become
available.
Note however, that you can still use to Equation Editor 3.0 by selecting
Object from the Text section of the Insert Tab and then select Equation
Editor 3.0 as the type of object to be inserted.
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Hope this helps.
Please reply to the newsgroup unless you wish to avail yourself of my
services on a paid consulting basis.
Doug Robbins - Word MVP