WordPress MU

I have installed wordpress MU for the first time version 2.7. The main MU admin loads up fine and I created several blogs. I then wanted to administer a particular blog on clicked on the 'back end' link to administer the particular blog I got a 'Error 404 - Not Found' message. When viewing the blog itself not the admin section I get the default wordpress theme and text. It is just the admin area I cannot access. Any suggestions as to what I need to do?

are you using sub-domains or sub-folders? Did you do all that on the same machine? I have had some issues when logging into sites on the same install, because when I login my FF remembers too much stuff and send me the wrong place!