Device mapper is in 2.6.9 and later, so you just need to make sure it is enabled either as a module or builtin to the kernel. Look for /sys/class/misc/device-mapper or look in /proc/devices for a device-mapper entry to see if it is enabled. If neither are there, try modprobe dm_mod, then check again. For versions previous to 2.6.9, either you or your distro must patch the kernel to support it. Check the device mapper web page for more information.