Yes, as guv1976 said, it could not be plainer. You must decline the rental company's CDW for your credit card to provide coverage.

That said, I must bring up some points. That card's CDW benefit is secondary to any existing coverage you might have. That means if you damage the car, you will be required to file a claim with your own auto insurance company first. You will probably be left with a balance to pay consisting of your insurance deductible plus any additional fees your insurance company doesn't cover (loss of use, administrative fees, towing, etc.--check with your insurance company on what they cover). Note that this claim may cause your personal insurance premiums to go up.

It is this balance, then, that the MasterCard underwriting company will consider covering.

Trust me: it's a lot of hassle, and I have seen many, many people let down by their insurance and credit card companies and left paying sometimes-substantial balances. It is for this reason I typically take the rental company's CDW myself (which is expensive and, as jbcarioca said, supposed to be a profit center for the rental agency, although I have seen it be a loss center in some years!) or at the very least rely on (for longer rentals, where the cost of CDW would be prohibitive) the paid protection available through American Express, which is primary (and thus does not require you to file a claim with your own insurance company).

It's all about what your risk tolerance is.