I have the same problem as the person who previously asked the question. Asking again because the issue was not quite resolved. I can access pset7/login.php but with /phpMyAdmin I get the forbidden message. All the recommended steps tried.
Eureka!!!! I looked up http://cdn-local.cs50.net/2011/fall/psets/7/pset7.pdf, which I think relates to an earlier version of the cs50 appliance, but it said try and access //localhost/phpMyAdmin, It seems to have worked.