To import a material data file, use the File/Import/Material data file menu item.

File → Import → Material data file

In the resulting Choose a file panel, select the appropriate .mat file and click Open (or double-click the file name). You will then be prompted to specify the simulation where you want to store this material data file. Ansys Polyman will store the file in the MaterialData subdirectory of the appropriate simulation subdirectory and create a leaf named filename.mat under Material Data File(s) in the relevant branch of the Simulation(s) section of the project tree.