A new feature of OS X, "Ignore accidental trackpad input", must be disabled in order to play the game with the trackpad.

 

This feature inhibits trackpad movement when a key is pressed, and can safely be turned off without distrupting your system.

 

To turn off this feature, open System Preferences from the Dock, then go to "Keyboard & Mouse" :

 

 

Next go to the "Trackpad" section, and ensure the section "Ignore accidental trackpad input" is unchecked:

 

 

That's all. Now you can control the game properly with the trackpad.

 

( Note: it is easier however to play the game with a mouse.)