When tabbing in the native Mac version of TaxCalc you may sometimes find that "tab" does not go to to certain fields
There is a setting in OS-X that defines the behaviour of the tab function
Click System Preferences - Keyboard (from the hardware section).
Towards the bottom of this dialog you will see the radio button for the tab behaviour.
Select All Controls to set the behaviour as expected. Alterntively you can use ALT+Tab and all fields will be tabbed to as expected.