After installing Drush on Mac it only downloads drupal 6x
Drush 7.x doesn't only download Drupal 7 of course. Drush 5 will download Drupal 7 too. Drush 7 is required for Drupal 8 and needs to be selected when installing with composer global require drush/drush:dev-master. Ps the recommended method is the composer method. This what I use on OS X 10.9.

Find old versions of drush with which drush. Check where they live withls -l [path from which drush]. Rinse and repeat until the only one left is in your home folder inside the hidden composer directory.

Check the version of the installed drush with drush --version

Make sure you don't have a .drushrc somewhere with a default core version specified.