Opening the PT11.1.3 Update

I have downloaded the PT Update 11.1.3 twice once cause it stalled at 97% and a second time not using the download manager but Firefox. So I tried to open the dmg file but I am given the warning it "can not recognize the file" and wont open. Any suggestions, I tried using DiscImage Mounter as a default which usually works no problem but with this it's a no go. This downloading is taking a lot of time and ADSL is limited to 30Gb a month.