General information about how to get and install MuPAD are given in Installation of MuPAD. You can install MuPAD on Linux by following the general instructions, but it is easier to use the slackware pkgtool as descripted below.
MuPAD for Linux is also available on sunsite.unc.edu:/pub/Linux/apps/math/MuPAD. Before you try to get it, please refer to the mirror list to find the ftp site next to you.