How do I add cells together on OS X numbers app
The error at the bottom of your screenshot is from a previous entry. The Result bar does not update as you type—it updates when the cell contents changes. There's nothing wrong with that formula. Press Enter to enter the formula into the cell.

                                                                 

As for adding cells: whilst in formula edit mode, click the cell to insert its reference into the current cursor location, and use the standard syntax (something+something).