The localDB instance should start up when you first try to access the database. After a few minutes of inactivity, the instance will close and only start again when you try to access the database.
There should be no need to "manually" start localDB, although I have noticed sometimes that the application does not automatically start, though this is intermittent and I can find no explanation for it.
EDIT
In order to try to start LocalDB when the application first loads, put this in your Program.cs file, in the Main() function:
Process.Start("sqllocaldb.exe", "start v11.0");
This will attempt to spin-up the localDB exe on application startup.