First you need to make sure the LVM tools are installed (package
lvm2). With grml that should already be the case. Then you follow this procedure:
Check the system for existing volume groups:
Activate the volume group with the logical volume you want to mount:
vgchange -ay VG_NAME
vgchange -ay (without a volume group name) to activate all volume groups.
Check that the logical volume is present:
Mount the volume:
mount /dev/VG_NAME/LV_NAME /mnt
If needed use
chroot to change the filesystem root while you work on the mounted volume.