If you have an unactivated phone and are using wifi you may have difficulty with Play Store. To get around this issue go out of airplane mode for a few minutes or even a few seconds, generally, to allow the phone to get the correct time from the carrier. This worked on Sprint with an Evo 4G for me. Play Store apparently needs the correct time. When you go out of airplane mode your phone will sync time with the carrier... Even if you don't have an account apparently.