PUT THAT MAC ADDRESS UP!!!!!!!!! BAD PRACTICE; though, I AS well did have your same problem. MAKE SURE TO TYPE IN THE CAPTCHA correctly, if this fails, refresh the page, continue from the start with your [non-existent] by now MAC address. This should solve your problem.
*Note: Because your MAC address was probably seen by others, I would be weary about who to accept the package from, as there may be a very... malevolent package to hack YOUR wii, thru another's PC/other media. Make sure this doesn't happen again, only worrying about yours and your wii's safety!