Try this advice from Andrew Chiang [MS]:
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Go into your ~/Library/Preferences/Microsoft folder and delete or move (to
another location) the two files below. By ~, I mean your home
directory/folder.
com.microsoft.Graph.prefs.plist
Microsoft Graph Toolbars (11)
Afterwards, try re-inserting your graph and typing in the cells.
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On 3/25/07 12:50 AM, in article
1174801802.077480.203010@p77g2000hsh.googlegroups.com, "woolie"
> Hi,
>
[quoted text clipped - 5 lines]
>
> I have never had this problem in Windows, so I am stumped...

Signature
Mickey Stevens (Microsoft MVP for Office:mac)
Office & Mac Resources: <http://home.earthlink.net/~mickey.stevens/>