First you need to Boot into a Live CD environment and open up a terminal window. (applications-->accessories--->terminal)
Install required packages using the following command
sudo apt-get install lvm2 cryptsetup
probe required module using the following command
sudo modprobe dm-crypt
setup the crypto module to recognise the partition
sudo cryptsetup luksOpen /dev/hda5 crypt1
Enter your passphrase. You should get the following message:
key slot 0 unlocked.
If not, something has gone wrong.
Scan for volume groups
sudo vgscan --mknodes
sudo vgchange -ay
REMEMBER the name of the volume group, as you will need it later.
Create a mount point
sudo mkdir /volume
mount the encrypted volume to the mountpoint you just created.
sudo mount /dev/paulb-desktop/root /volume
The volume is mounted, now you can chroot or whatever else you need to do. If you would like to open the gnome file manager for writing to it issue the following command:
sudo nautilus /volume