I have created a database \"test\" with some tables in MS SQL Server 2008 R2 (i.e. MS SQL Server Management Studio).
Now, I need to export this database as a MDF file.
C:\Program Files\Microsoft SQL Server\MSSQL10_50.MSSQLSERVER\MSSQL\DATA\
This is the path where you will find MDF file
If you are in Visual Studio, go to SQL Server Object Explorer. Find the database you want, right click and select properties. In the properties tab, look for Data File, next to it will be the location for your mdf file.