There are many different routines in MRST for generating unstructured meshes. You can read about this in Chapter 3 of my textbook, which can be downloaded for free from the publisher:
https://doi.org/10.1017/9781108591416
If you, by unstructured meshes, refer to Voronoi (or PEBI) type of grids, there is a lot of functionality in the
upr module, whose functionality is described in Chapter 1 of the second MRST textbook:
https://doi.org/10.1017/9781009019781.005