To add arrow keys to your primary keyboard pane, go to SwiftKey settings->Theme & Layout->check arrow keys.
See the how-to video here.
For Tablet users:
The above option does not exist.
The best way to do this would be to select the extended layout. To do this, go into SwiftKey settings->Theme & Layout->check Extended layout. This will offer left/right arrow keys on the primary keyboard pane.
You can also access arrow keys when using SwiftKey on your tablet by tapping the 123 key in the lower left of your keyboard.