can't install WP locally on Mountain Lion

I upgraded to a new iMac running Mountain Lion and 3.5 for WordPress at the same time so I’m not sure where the problem lies. I’m also using MAMP 2.1.1 as my local server.

I go through all the usual steps of creating a locally hosted site — creating a database, user, etc. but when I try to install WP by going to the root (for example mysite.dev), instead of getting the page to add site name, user name, etc. I get a message from the browser asking how to open the file. I’m mystified. I can run other sites using PHP but I haven’t tried any other CMS’s like Drupal.

I’m totally stumped — would be grateful for some help with this.