When loading date fields via Data Manager, these will be automatically recognised and tagged as date fields. If you explore the auto-generated load script you'll see the following expressions for each date field:
Date(Date#(DateFieldName, 'DD/MM/YYYY'), 'DD/MM/YYYY') as formattedDate
If still date picker object doesn't display/recognise that field, then try this trick:
Select one of the available fields from the drop down
Enable 'Advanced Setup' (i.e. 'ON')
Use the expression editor to enter the desired field manually.