Mubi won't help you. They don't answer customer enquiries and they don't give refunds... make sure you've cancelled with your bank or Paypal or they'll take a subscription every year... they've made a lot of money that way. I can watch them on my Macbook Pro, but usually stay watching Youtube alongside whist waiting for them to buffer. If it's a halfway popular film you may never get to the end. Also they bring the annual payment date forward by a few days each year to catch you out.

I am a PC person myself, but I suggest trying the free Divx install. Its only a "demo" but playing all formats is a totally free part of DivX. I have, in the past, solved numerous persons' problems with the DivX install