What format should the cell be? This can not be added and only gives a exclamation mark - "the formula contains a syntax error. OS X and numbers v3.5.3
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).
gagarside's answer is the correct answer.
To take this a step further, you could use the
To sum values hard coded (like in your original post):
To sum values in cells (more dynamic as one can change single values easily):