you must first install the latest SDT release. Unless we suggest otherwise, you should use the pre-release version at https://www.sdtools.com/sdtcur_dis.html).
You should use a 64 bit MATLAB (it is not realistic to use viscoelastic tools on a 32 bit application)
install the viscoelastic tools by patching your SDT.
in MATLAB, move to that directory and install the patch simply by using
visco_patch_dis. Note that you must have write permission to the SDT directory so that if you have installed SDT in the "Program files" directory you should run MATLAB as an Administrator, see https://www.sdtools.com/faq/Release.html#install.
check that the documentation has been installed using sdtweb('visctoc').
if you intend to use larger model install the SDT/MKL server using sdtcheck('PatchMkl').