It is a common problem that windows 8 Apps/Store don't work while Using a
Proxy.
What we have to do is to just bypass your Proxy address and Port to the Apps.
Follow these steps:
Step 1 - Go to CMD (command prompt). Run it as Administrator(right click
and choose Run as Administrator)
Step 2 - Type following commands.
netsh
winhttp
import proxy source=ie
exit
This command imports the proxy address from your internet explorer to your
Apps/Store.
Enjoy using Windows 8 Apps.