You do have a dilemma here.
B&N and others (Apple) use the credit card to identify where you live so they comply with copyright and/or licensing issues for the content they sell (DRM). In your case B&N does not offer their Nook for sale in your country.
Here in Boston we have a lot of students from all parts of the world. They often end up getting a US credit card to pay for things here locally. As they too encounter issues paying in Dollars Vs there local monetary exchange and also face issues with the same DRM issue when they are here.
One other thought here - I do believe B&N offers pre-paid cards at local North American stores that offer them. You could get around the issue buying one of these cards here in the states (if you are here). Sorry to say you'll then need to purchase a new pre-paid card every time you run low in funds (maybe you know someone in the states that can do this for you).