Hey!
As of STM v1.5, multi-material UI is only supported on Unity 2017.1+, as it would cause some nasty crashes in older versions, so make sure you're using that, first. Otherwise, you'll have to use the non-UI version of STM to use multiple materials.
To switch between multiple materials, go into TextData (click the T in the top-right of any STM inspector) and find the "Materials" drop-down. Define a new material there, the name you use here will be the string you use to call it. Then, in your mesh you can use the tag <m=materialname> to switch, and </m> to return to the default material!