Where can I switch on the Access code field for the learner?

You can do this at the Settings page. Here you go to Advanced settings and you will see these options:

Access code

When the checkbox Access code is checked, learners can use access codes to gain access to a new course. When the box is checked when logged out the access code block is also available on the login screen. This subsequently means that new users have the possibility to create an account first with their access code. This option only has to be turned on if you would like to subscribe learners with access codes.