There is no way that I know of to buy disney dollars with flooz.
With a webcertificate, you will need to call them and order the plastic card (it's a $6 one time fee charged to another credit card) and then take the card to your local
disney store to buy the disney dollars.
Other ways that you can use the webcertificate without buying the plastic card is by paying on your room or package reservation. You don't need the plastic card to do this. If you have booked with an online agent like
Dreams Unlimited (that is who I am using) just enter the info like a credit card. Or you can call and make the payments over the phone if you made your ressies straight through disney. Just tell them that it is a mastercard and give them the info. Or, if you are planning on buying anything online, such as film, clothes, luggage, etc.. you can use your webcertificate to pay for it