Using PowerPoint 2004, You can't edit the data in a Microsoft Graph object in your PowerPoint presentation.
Solution
Try this advice from Andrew Chiang [MS]:
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.
Did this solve your problem? If so, please consider supporting the PPT FAQ with a small PayPal donation.