This fix is necessary for systems that run a 64 bit windows system (candidates are Vista, Windows7, Windows8, …). You can check your system version by following the instructions on this page (second option down).
In order to fix this issues you have to exchange a files in the Wherigo builder folder (c:\programs(x86)\Groundspeak\Wherigo\Builder)
Should you receive an error message during your download attempt, you need to log in to the Groundspeak forum. An option to do so is provided on the error page.