Enter the part number and description and ensure the 'Type' is "Assembly/SubAssembly" - see the image above.
MiniMRP includes a type of assembly "CSAS (Closed Sub Assembly)". Use this type if you're creating a record for a sub assembly that is generally produced and held in inventory in advance of requirement.
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.