Enter the part number and description and then - change the Type to "Assembly/SubAssembly" - see the image above.
In previous versions of miniMrp the category must be "AS". In this version you can use/create any category name.
Click the 'Save' button and exit.
Now that you have an assembly you can use the Parts List Editor to create a parts list.
Select 'Assemblies' from the main menu and miniMrp displays a list of all assemblies in your database - see below.