You need to use Mandrakes own tools for managing packages. Apt and Synaptic are not supported by Mandrake (officially anyway) because they have their own tools (urpmi and rpmdrake) that do a similar job. You need to configure urpmi sources for your Mandrake release. Go to
zebulon.org.uk to find out more about urpmi and
easyurpmi for a guide on how to set it up on your system.