I get an error saying that there was a problem parsing the package.
This because your app is not compatible with your device. It might your developer set the minSDKVersion value to high and If your device API level is lower then minSDKVersion then it gives parsing package error or your developer set the maxSDKVersion to lower than your device API level.
For ex : If your device is Android 2.2 then your developer who made the APK set the minSDKVersion to 10 then it wont work in the Android 2.2 and will give the parsing package error or If your developer set the maxSDKVersion to 8 then app won't install above Android OS 2.2 and if you try to install then you will get the Parsing package error.
