I wrote:
are you logged in as User 1 (the site administrator's account). If you are logged in with any other User ID it's possible your permissions are preventing you from seeing that page? As an experiment, try going to http://www.example.com//#overlay=admin/modules (for Drupal 7) or http://ade.pt/admin/build/modules (for Drupal 6 ... probably the same for Drupal 5). What do you see?
um, that 2nd example was supposed to be: http://www.example.com/admin/build/modules (for Drupal 6)
I had forgotten to replace the "ade.pt" of my own domain with "www.example.com" (which of course you'll replace with your domain)