How do I make a reservation if my Present Balance is insufficient to make the reservation?

You have two choices:
1-Add a payment method if you do not have one and Buy the deal. In this case you will not need the Reservation Deposit and you will still earn the number of DealCoins for that offer when the deal is closed.
2-Ask someone to transfer DealCoins to you as a loan. Once you close the deal the deposit will be returned and you can transfer back the amount you borrowed.