Tina (and Usman), today I decided to try installing this MAMP program on my Mac. I had the same difficulties as others, and made it down to your post. When I followed your advice to go back to 8888 and 8889 after changing -envvars, yep, everything worked form me too! Thanks for your post.
Quite honestly, I have desired to have the ability to run an experimental version of Moodle on my local computer for about 5 years. For several years on my PC, I never seemed to have success. Now finally on my Mac, I think that I have this ability.
Soon, I am going to see if I can install Moodle 2.7 into this environment, update 2.8, and finally install a version of Moodle on my Mac to mirror my production web server's install. Can't wait!
Thanks again.