Note: Refer to the answer on Stackoverflow (http://stackoverflow.com/questions/17833232/cannot-connect-to-localhost-in-windows-store-application)

By default Visual Studio adds an exception in the NetIsolation which permits and denies local loopback/localhost access. You need to add your app as an exception in this list, however the information in the link provided in the OP doesn't work, I couldn't get the command to work at all, so here's how I did it:

Go to your registry, and CTRL+F to find your app (for example if it's called TESTApp1, then enter that in to the search box).
You should find a registry key that looks like a guid followed by a name, for example:
ac2efce7-a15b-40d0-92db-3abde43a6778_1.0.1.15_neutral__025mzc78q1aqe

Once you have that name, in a command prompt (as administrator, just in case) run :
CheckNetIsolation.exe LoopbackExempt -s


In the output, find your GUID. You will see the SID under it, make a note of this SID. 3) Run the following command, entering in your SID on the problem machine:
CheckNetIsolation.exe LoopbackExempt -a -p=SIDHERE

Note: Alternative step 2: You can also find your SID under the following registry key (assuming you have installed the app):

HKEY_CURRENT_USER\Software\Classes\Local Settings\Software\Microsoft\Windows\CurrentVersion\AppContainer\Mappings

Again, CTRL+F and find your app under there.