Cannot add fields to content types

I'm using Drupal 6.4 with CCK 6.x-2.1. Under 'CCK' in on the Modules page, I've enabled Content, Content Copy, Fieldgroup, Number, Option Widgets & Text.

I'm able to create a new content type. However, when I click on 'edit', I get no tabs (I've tried with multiple themes) and I see no section for creating a new field. Same thing happens when I click 'manage fields'.

I've tried disabling all other modules, but I still can't find a way to add custom fields to my content types. Could I be interpreting the documentation incorrectly? I've tried to think of anything else that could possibly be going on, but can't come up with anything.