Actually you cannot use a PNG mask in My Memories Suite for photos, usually the user has used a different software program (like Photoshop or Photoshop Elements) where they clip the photo to the mask. Once they do this they save it in PNG format and then bring it into MMS as a png file under embellishments. So, if you have a program other than My Memories Suite that works with PNG files you can use that to clip your photo to the mask and then bring it into the MMS software.
But what if you don't have other software? I tried to figure out a way to allow MMS users to use a mask without having to leave the program using what I call Mask Packs or Fancy Edge Frames. Here are instructions with pictures to help you understand how to use the Fancy Edge Frames/Photo Masks with Memory Mixer and My Memories Suite. Basically the mask is cut out of a piece of paper and you place your photo behind it. This allows MMS users to get the look of a mask.