What 3dsMax version are you on? It looks like you're using windows 8, is that correct? I'm developing on a win7 machine and can't test if win8 performs differently. And could you try to log into Sketchfab with Internet Explorer (that's what the popup is supposed to be) and tell me what happens?
Finally, if you're brave, you can check your registry. Open the registry editor and find the following key:
In there there should be a list with application names including "3dsmax.exe" it should have a value of 10000. Can you confirm that?