I had this problem also and it was a pain. I configured my connection string and managed to solve the problem. In the connection string I replaced the value |DataDirectory|\dbfilename.mdf for AttachDbFilename property, with the path to the file. |DataDirectory| can only be used if the database file is in the App_Data folder inside same project.
So changing the AttachDbFilename property to the direct path of the mdf file, fixed my issue.
AttachDbFilename=C:\MyApp\App\DAL\db.mdf
I hope this works for you.