If you have already used Manage | external data sources to define the external data source....
Open Manage | Database | Relationships.
Click the button at the very far bottom left in this window to create a new Tutorial: What are Table Occurrences?.
Select the desired table from the list of tables in the scrolling box.
Name your table occurrence and you now have a table occurrence that refers to your table in the external file.
You can use New Layout to create a layout that refers to it.
If you hadn't already used Manage | External Data Source to create the external data source reference, you can skip that step and use Add FileMaker Data Source from the Data Source drop down in the top of this dialog.