How to print a mirror image using powerpoint from my macbook to hp printer

I am making t-shirts and I can't get this image to reverse (print out as a mirror image, which I need for the photo transfer paper to work I can reverse part of the image using the rotation tool for "flip horizontal", but for some reason it doesn't work on all of it. Also, I'm using a macbook and when I go to print, under "paper tray/quality" I can actually see a box to check for mirror image, but it is in grey scale, so I can't select it.