NIntendo has put up a new tool on their site where you can put in your 3DS Serial Number and verify whether you are a Nintendo Ambassador. Here's the information straight from the site. Note: Some people have been getting messages saying that they are not Ambassadors when they put in the whole code. If this happens to you, try putting in all numbers except for the number in the box. I tried it both ways and it worked for me.
Nintendo 3DS Ambassador Eligibility To verify that you are eligible for the Nintendo 3DS Ambassador program, enter the serial number in the box below. The serial number for a Nintendo 3DS is located on the bottom of the system - just below the barcode - and starts with the letters CW followed by 9 digits. (Be sure to include the last number in the box.)
I put it in without the CW, and I was getting undetermined both ways. Then I tried it with the CW (silly me for thinking since they all start with that, that it would be just the numbers) and it works. Mine works WITH the boxed number, by the way.
Seems like this tool is a piece of shit based on all the issues people are having.