If you are testing your app in Xcode iPhone Simulator access the following folder:
~/Library/Application Support/iPhone Simulator/User/Applications/.../Documents
Trouble to find user's Library?
Navigate to users home using Finder, click to Go from menu, then go to folder...
and type ~/Library
Hmmm... and when you testing your app to iPhone?
Use the Organizer from Xcode
Click your iPhone under Devices, then in the Summary section find your app under Applications. Click the triangle next to your app which should reveal your Application Data with an arrow button to download it to your Mac.
Let's code now..
For the second solution credits to Rob