You do it the same basic way you would file Filemaker 5. What's different is that you now have something called a relationships graph to define relationships and you can put more than one table in the same file if you want.
Define a relationship linking your two tables if you haven't already. (I'd give you step by step but I skipped that version. The help file can probably tell you what you need to know here.) You can link tables in the same file or different files.
Place the product description field from Product Master on your acounting layout. If the relationship is valid (I'd guess you have a product ID field in both tables to use in your relationship) you should see the product description field appear and display the description for the product recorded in the accounting record.