How to install google apps when android built from source
There is a lot more to the Google Apps than just downloading and installing the APK.  Google's Services framework and apps (a.k.a. GApps) are not part of AOSP which is probably where your source comes from, but they are required for Play Store and other apps to work properly.

Unfortunately GApps is closed-source, so it's not allowed to re-distribute these portions.  However, since your Nexus 7 tablet shipped with them, you should be able to easily find all the necessary components in the original image.  Alternatively, you should be able to find the GApps package for your specific Android OS version that works on your tablet over at XDA-Developers forums.