I recently wrote a blog post about Outlook 2011 for Mac showing my name as "Default User Nam" in replies. I had no idea why Outlook 2011 for Mac was using "Default User Nam" so I presumed that, in keeping with the name itself, it was a program default. But then I noticed something: when any of the Office 2011 for Mac apps loaded on my machine, they showed that "Default User Nam" in the load banner. Maybe that's what you get if you install Office 2011 for Mac and don't put something else in--but maybe it's just what ended up in there specifically in my case when Adobe IT did this install. But in any case, it makes one thing pretty clear: in my case, the name that shows on the load banner for Office 2011 for Mac programs (you might also refer to this as the name used to register Office 2011 for Mac) is the name that was used to create the initial contact for "me" in Outlook 2011 for Mac. So, for what it's worth, that's certainly how the issue I described in my referenced blog post above (the issue of Outlook 2011 for Mac showing "Default User Nam" for me when writing an email reply) came about.
Now, if you want to change the name to which Office 2011 for Mac is actually registered, check out this forum thread. This will change what appears in those Office 2011 for Mac load banners. However, I strongly suspect that this won't fix the issue with Outlook 2011 for Mac using the incorrect name for you when you write a reply--but the good news is that the instructions in my blog post linked above will help you fix that issue. And note that while in my case there is strong evidence that the two issues were related, they don't have to be. So you might have your name properly entered for Office 2011 for Mac such that it shows up correctly in the load banners and yet you still might be getting the right name showing up for yourself in reply emails--well, that's okay: my blog post linked above will still help you solve that issue. It doesn't matter if you're getting "Default User Nam" as the wrong name for yourself when replying to email or if you're getting some other random wrong name or even if you're getting the name of one of your real contacts: it's all ultimately the same situation and going through the instructions on my blog post linked above will help you.