The choice list field needs to have choice options setup in order to display them for the user. There are two different options when creating choices: 'Specify choices here' and 'use a pre-defined choice list'.
When 'Specify choices here' is selected you will see a box labeled 'Choices' where you can enter in the choice options you wish to use for that choice field. Each option should be entered on its own row within the choice list box.
You can click the 'Sort A-Z' button to have the choice list sorted alphabetically, otherwise, the list will be presented exactly as you entered them in the choice list box.
If you wish to add values for the choice options you will want to use a semicolon between the label and value for each choice option.
The 'label' is what the user sees when filling out a form, and the 'value', added after the semicolon is what is included in the exported data and pulled into calculation fields.
When collecting data on the Mobile Apps, users will tap the field, which will take them to a new view with the options entered in the step below.