If you don't see libc6-xen 2.6.1-1ubuntu10, then that's because you don't have gutsy-updates/universe in your /etc/apt/sources.list. You need lines such as these (though please use an appropriate mirror instead of archive.ubuntu.com if possible):
deb http://archive.ubuntu.com/ubuntu/ gutsy-updates universe deb-src http://archive.ubuntu.com/ubuntu/ gutsy-updates universe
If you don't see libc6-xen 2.6.1-1ubuntu10, then that's because you don't have gutsy-updates/ universe in your /etc/apt/ sources. list. You need lines such as these (though please use an appropriate mirror instead of archive.ubuntu.com if possible):
deb http:// archive. ubuntu. com/ubuntu/ gutsy-updates universe archive. ubuntu. com/ubuntu/ gutsy-updates universe
deb-src http://