Please contact Support to switch this feature on for you. If you would also like to make it required (i.e. the user MUST fill in the data field to save/update, then you can ask for this also.)
To modify the fields of a Inspection, first find the inspection or click to add a new inspection.
Once you have created/found your inspection, custom fields can be added on the bottom left of the Info tab of the Inspection.
To add a new field, click the pencil icon and you'll be able to choose from all fields you've configured across your entire database to add values for, or create a new one.
You can choose to create a new field, and also choose whether that field is searchable. If you make a field searchable, it will be able to search on your Inspection list and Filters.
You can filter any custom field created on the left sidebar of the Inspections tab
You may also access data stored on your inspection fields via your Templates. Read about Template Merge Tags here.