In iOS 7, the document directory of the iOS simulators can be found in:
/Users/Sabo/Library/Application Support/iPhone Simulator/
However,
With iOS 9.2 and Xcode 7.2, the following script will open the Documents folder of the last installed application on the last used simulator;
cd ~/Library/Developer/CoreSimulator/Devices/
cd `ls -t | head -n 1`/data/Containers/Data/Application
cd `ls -t | head -n 1`/Documents
open .
To create an easy runnable script, put it in an Automator Application with "Run Shell Script":