To Enable NAT For An Interface:
If you have selected 'Automatic' option, rules will be automatically configured for all configured networks that does not have gateway configured.
If you have selected 'Manual' option, rules will be added same as the automatic option and you will need to add/edit to suit your needs. Pl. note that rules will be added only the first time you switch from 'Automatic -> Manual' mode.
To add a new rule click on the 'Add a new rule' button.