There is no app for Matrix, so there is nothing to download or install.
Just open the browser on your phone and go to www.smartmls.com.
Enter your ID and password and tap MLS Login:
Tap on the Matrix button in the MLS Products section:
That will bring you directly in to Matrix Mobile.
If you prefer to work on the full version of Matrix (exactly what you see when you are on a PC or laptop) tap on the Full Site link at the bottom of the screen.