Don't know if you knew this already or not, but the "boxes" in Manage | Database | Relationships are called "table occurrences". You can have many occurrences of the same table if such are needed for your solution.
In Manage | Database | Relationships, click the button in the bottom left corner. This creates a new table occurrence and you can then select an existing data source table from the drop down of available tables for your new occurrence in the dialog that opens when you click this button. If you had not already gone to Manage | External data sources and created the external data source reference, you could have selected Add FileMaker Datasource to create one. Since you have already done this, you should see the tables from this external file included in the list of available tables.
You've saved me again. Your advice was just what I needed. I'm not sure how to explain why that didn't occur to me. I'll guess I thought the "Add a table" button was intended to create a new table rather than to add an existing table occurance to the relationship diagram.
By the way, do you know why a number of the tables in the "Business Productivity Solution" files have data elements (such as "ID" fields) that are defined as "text" and as "Serial Number on creation"? I would think that anything that holds an auto-generated serial number would be a number rather than text.
Whether or not a serial number field is defined as text or number is the preference of the developer. Like you, my preference is to put numbers into a number field--you never know when you might need to sort on that field and numbers in a text field don't sort correctly. But as far as uniquely identifying records in a table for use as a key in a relationship--either field type will work.
Belated thanks for the explanation. That confirms what I suspected.