To add a field on the Registration Page, you will first need to create that Custom Field in People. Any of the Custom Fields in People can be added to a Registration Page. Go to People > Custom Field > New Custom Field to create a new Custom Field.

Once this new field exists, you will be able to go to Webmaster > Pages to see your list of website pages. Click on the Registration Page you are designing. Then click on the blue edit icon Add Fields and scroll down to choose the new Custom Fields you have created. Click Save and Save and Publish in the upper right of the screen when ready.