which version of Outlook you use, and I'm using a beta of the Office 11 version of Outlook, so bear with me.
In the inbox, go to 'Actions'and select 'junk email' and then 'junk email options'.
There you should see a list of the addresses you've nominated as junk email senders, and you can remove the one you added by mistake.
In the new version of Outlook there will be a special icon, labelled 'Not junk' which will enable you to rapidly relabel anything you (or Outlook) have mistakenly identified as junk. I don't think this is in any of the presently available versions.