Are you located in the US? If so you need a US RN license, and can find test centers by visiting http://www.goamp.com
Are you located in the UK? Your licensure is equivalent to a US RN? If so, you need to contact the BCEN for information on an alternate test site. In the US the test is computer-based - for international candidates
"Candidates living outside of the U.S. must contact BCEN to
determine what testing options are available. Alternate testing
requires a 90-day advanced scheduling notice."
Are you US Military living over seas? If so, talk to your DANTES coordinator - not only might they be able to help you set up the test, but they just might pay for it.
Here's the official brochure for the CFRN examination: