You can go to IRD in person or write a letter for apply the instalment of payment of the salaries tax and the interest will be applied. You can write in the letter as the ground that you are now unempolyed and apply for instalment for the tax payment.
You can bring up your financial info such as bank statement, passbook and goto Wan Chai IRD HQ to apply for settlement by instalment / refund of previous balance (if any) and asked for help. I perfer if you can go in person is better in this case.