i can register my mac. says serial number is incorrect

I don't know why the serial number from the About This Mac window does not work, but you can try looking for and using the serial number that is physically printed on the surface of the Mac. This acticle explains where to find it.

http://support.apple.com/kb/HT1349

Make sure that the letter S and the number 5 are properly noted.

Best.

If you have an invoice from the Apple Store or an Apple reseller, check if the serial matches with the one printed on the Mac.

Later contact Apple Support and ask help.