As a general rule of thumb, it is "bad karma" to store MDF files anwhere other than with the server's primary store of MDF files... Having the MDF files scattered all over your filesystem makles it difficult to protect and manage them.
If you wish to fight for your right to cause yourself ongoing pain, we can accomdate that desire!
Check to see what Windows (or possibly Active Directory) account is running your instance of SQL Server. If you aren't sure how to do that, I can give you detailed directions.
Use SSMS to see where the disconnect happens when looking for your MDF file in its present location. You can probably see the drive, and one or more folders, but not all the way down to the MDF file itself.
Once you know what Windows user needs permission and where in the folder heirarchy the existing permissions peter out, you can then use Windows Explorer to grant the needed permissions for the Windows User the runs SQL Server.
Before I go, let me emphasize that this is a really bad idea. Only pursue this idea if you are really intent on causing long term problems while you use this database.
In theory, theory and practice are identical. In practice, theory and practice are unrelated.