Cacheator,
I am currently using IE 11 with a Windows 8.1 PC and have had no problem logging in to anything nor downloading any of the Wherigo Cartridges. I do have Wherigo.com added to "Compatibility View Settings" located in the Tool drop down.
Setting Compatibility View for some web pages has been needed since the advent of IE 8 and isn't a big deal if one wants to still use IE for browsing. While other web browsers may do the compatibility automatically, Microsoft decided to force the end user to set the flag in hopes the end user would complain until the offending web site changed to the W3C standards Microsoft wants everyone to follow.
Also, going to the IE Advanced dropdown tab under Internet options and checking the box labeled "Use software rendering instead of GPU rendering" will correct other problems one might be encountering while using IE 11.
Hope this helps you with your download and login problems.
Al