When trying to upload numerous changes to your account or import campaigns from the other search engines, make sure to use the template MSN provides and that your columns are in order. Too many or too few columns and the upload won't work.
Forget about using the import feature for uploading negative keywords. In Joe's experience, adCenter just took the keywords he listed as "negative" and incorporated them for broad matching--an error that could cause a major headache if you don't catch it. AdCenter also doesn't play well browsers other than IE, so don't attempt to use FireFox or Safari when uploading your CSV/Excel files.
Lastly, make sure that after you've uploaded your campaigns you select which countries you want to target, as adCenter automatically sets them to run across the entire network (well over 100 countries).