You do not directly "put" data into MDF file, SQL Server/Express does (that
is, MDF file is part of SQL Server/Express).
In your previous post, you said you have a bunch of table/data in SQL Server
Express' master database and want to export them. Master database in SQL
Server is used by SQL Server itself to hold SQL Server's information itself,
not user's data. If the mentioned tables/data is your table /data, it must
be in a different database, not in master database of SQL Server.
If what you want to do is to transfer your data in your local SQL
Server/Express to another production SQL Server, you do not export your data
into MDF file (the data is already stored in mdf/ldf file). What you need to
do is to do a full back up to your database on the local SQL Server/Express
(you must know which database it is: a SQL Server/Express can hold many
databases), and then restore the backup (saved in a proper media, CD, DVD,
HD...) to the other SQL Server. You can also detach the database from your
local SQL Server/Express, and copy the mdf/ldf to a media, and then attach
it ti the other SQL Server.