The custom fields are set up under the Car Park Entity and if a custom field is uses a keyword set then this must exist before the custom field can be created.
Navigate to Admin Settings > System Config > Custom Fields.
From the Entities list, select either Assets or Locker.
When you set the custom fields under the Lockers Entity, you can select the Visibility to be Visible on Lockers form and then the custom field will display in the Edit Locker Form's Custom Fields tab.
Custom Field Example
Pin Number
This custom field was created with the Data Type of Text.