Yes, add salt. The salt dispenser is not to soften the water, it is to periodically flush out a water softening device.
Also, if you are in a very hard water area, then when you have used them up, don't buy them again. Buy 'Classic' tabs, and add separate salt and rinse aid.
This was advised to me by an engineer who came to repair my six month old machine with a burnt out heater.
Our area is extremely hard, and he said the three/four/five in ones could not cope with that.