What version of Linux Mandrake do you have???
If you have Linux Mandrake 9.0 or 9.1 then boot up your linux system and login as root.
Then start Mandrake Control Center and open the boot loader settings
there add new entry and choose a name for opsystem and choose on wich partition or drive it is installed (if you have pultiple harddisks, the drive are named hda, hdb and so on and partitions are marked as hda1, hda2, ... ,hdb1, hdb2, ...)
Make sure that the boot device is set to your primary master (usually hda)
and you should use LILO graphical mode.
Save the settings. Now reboot your machine and you should see the op system list on startup.