The kernel of Debian 6 has no memory cgroup feature. However you can run lxc without it.
If you NEED memory cgroup, it's easy to install the new kernel from backports.
- Add apt-line of backports
- Run "apt-get install linux-image-3.2.0-0.bpo.4-amd64" (or -686 for i386)
- Add a kernel boot option "cgroup_enable=memory" to your bootloader setting (e.g. /etc/default/grub) to enable it.
- reboot
Or, if you'd like to re-compile the kernel, you can use kernel-package system of Debian; http://newbiedoc.sourceforge.net/system/kernel-pkg.html