I installed Windows Enterprise x64 RTM activated with kms . I logged in with my Microsoft live account. All Metro apps work perfectly except windows app store. When I click on it, it says :Windows store isn't available in your region. Of course I tried to change region to another region like United State and others, but with the same problem. Pease I need help or any workaround. P.S. my internet connection 100/100
Have you tried using a US proxy? If it's region locked then it would likely look more at where you're dialing in from than what region you have set in the OS. I haven't had any problems accessing the store in the Enterprise x64 RTM.
Check your time zone on the clock, region in the control panel, and if that doesnt work, do a refresh of windows 8.