Can't use SQL Server database on a networkpath
An MDF files is not the database, as such. It is merely the on-disk storage used by the SQL Server database engine.

When your app "uses" the MDF, it is actually attaching them to an SQL Server install and running queries against that. This is transparent really.

Instead of this, you can install SQL Server on the network (even Express edition), attach tehe database, and connect to that from all your app installations on the network.

This SO answer has way more details: https://stackoverflow.com/q/10366586/27535