Hello thaehn,
Your clue is really helpful! Now I can reproduce the issue on my Windows 8 machine.
I installed both Lightroom 4 and Lightroom 5 on Win8, and then uninstalled Lightroom 4 from the Control Panel, finnaly when I tried to setup a Flickr service in Lightroom 5, I got the same IE page like Tiger posted: "Look for a app in the store".
So I want to confirm with everyone of you, have you once installed Lightroom 4 on your Windows 8 and then uninstalled it before you catch this problem in Lightroom 5?
And my test result shows that this problem can be resolved if you re-install Lightroom 5 via the installer. Could you please try it and let me know if it works for you?