How do I add content type in taxonomy settings?
Drupal 7 and 8 Core does not have that. I've never used Drupal 6, based on your question, it seems like they changed how tax terms are added to content types in D7. 

For Drupal 8, all you have to do is go to Content Type (ex: Article) >> Manage Fields >> Add Field >> Select: Taxonomy Term >> Save Field Settings >> Checkmark your vocabulary >> Save settings.