Are you running Office 2011 and is it fully up-to-date? The DVD version currently being distributed doesn't have native retina support, but if you go to Help>Check for updates within the application there are a number of updates which will make it compatible with retina displays.
If you're using Office 2008 then I don't think there's an update to fix retina displays.
Just to let you know that sometimes when you do 'Check for Updates' it shows as no updates available. However if you Cmd+Q, then reopen and check for updates again, there'll be one that shows up. That's Microsoft for you, I guess.