If you are seeing a "Package File is Invalid" error when trying to download MapMyFitness or any other app from Google Play, you will need to clear your Google Play's data and cache. To do this:
- Go your device's Settings menu.
- Go to Applications (may also be labeled Application Manager).
- Tab over to All Applications.
- Open the Google Play Store app.
- Tap on Clear Data and Clear Cache.
- Relaunch Google Play and install or update the our app again.
Since this is an issue with the Google Play Store, for further support with this issue you will need to contact the Google Play Store customer support directly.