This problem stopped me too. I got past it though..
On the 1st 2 tries I ran the ubuntu install and then let it update itself. No-GO.
3rd time I did the ubuntu load. restarted and then went straight into the MCE install. I used the default options. It went through fine.
In this link http://wiki.linuxmce.com/index.php?title=Known_Issues&printable=yes at the end it mentions this issue and says that the repositories may not be avaiable all the time. the 3rd time lucky install that i did - did the MCE load late on sunday night so maybe this was the case and a low traffic time helped - but is think that MCE just prefers 6.10 as released.