Hi Sharksfan,
You can do this with a custom data validation on the attachment node. Double-click the node in the Data Source task pane, go to Data Validation tab, and then add a rule that is true when:
"Attachment node is blank" AND
"Checkbox is true"
Now, add a custom error message and you should be good to go!
Matt Faus / Microsoft InfoPath MVPQdabra® Software / Streamline data gathering to turn process into knowledge