It's a little bizzare. I thought Microsoft had a good single-signon system with .net passport. However when I use adcentre.microsoft.com I had to create a username, although advertising.microsoft.com uses .net passport
While (i'm sure) they are different products, it doesn't help that the link on live.com takes you to the one that doesn't manage my ad campaign for live.com !
I too had lost the link whilst at work today, and couldn't find a link back to it (you'd expect it to be obvious), lucky my home PC still had the correct one in the history.