Not sure about Mandrake src.rpm's, but I would guess you can simply install the rpm and you'll find the source wherever src.rpm are installed to (on SUSE, this is /usr/src/packages/SOURCE).
Another option is to extract the source.tar.gz from the rpm with rpm2cpio and cpio:
http://www.rpm.org/max-rpm/s1-rpm-mi...-rpm2cpio.html