To use custom subscriber fields, double-click the subscriber you want to change on the "Subscribers" tab to open the "Subscriber" window (see Editing Subscriber Entries), and click the "Custom Fields" tab.
Initially, there are no fields there; to add new fields, click "Manage Subscriber Fields":
Use the window to add new fields or delete existing ones. For example, type "Phone", and click "Add". Then type "City", and click "Add" (you can also press the "Enter" key on your keyboard after you type the name of the new field). Click "Close" to return to the "Subscriber" window.
Note: There is no way to undo the changes (no "Cancel" button) so if you make a mistake all subscriber records will be affected immediately. For instance, if you had a "Phone" field holding subscribers' phone numbers, and deleted it, there would be no way to revert it back.
As you can see, the new fields show up on the "Custom Fields" tab of the "Subscriber" window:
Note: By adding the fields, you change the entire mailing list; the custom fields are added to all subscriber entries (or removed from them).
You can type anything you like in the fields, and then click "OK" to save the changes and to close the "Subscriber" window.