Your release line is wrong, and you need to specify what of the branch you want to download:
Code:
*default host=cvsup.uk.freebsd.org
*default base=/var/db
*default prefix=/usr
*default release=cvs tag=RELENG_7_0
*default delete use-rel-suffix
*default compress
src-all
For most people, network bandwidth is the limiting factor, so I threw the compress option in there too.
There are examples of sup files in /usr/share/examples/cvsup