Have a look at the Tutorials or the presets Sample file on the Disc.
It may not make much sense but may be it has all been done for you.
There is a learning curve.
To give you a hint, Purchase Order, Product List and Invoice should all be separate tables and you will need a fourth table: Line Items. Relate your tables correctly in Manage | Database | Relationships and use a portal to LineItems on your Invoice layout to list each individual item on your invoice.
Could you give me the details to the Portal information?
I do already have a portal set up but apparently there is something within the portal that is not allowing the line items to show a list.