Okay. So here's what I'd do. I'd add a data connection to the list. Then you can use a simple XPath expression for your filter. I'm attaching a simple sample form - save the linked file locally, right click and select design to open in design mode. All my data in the sample come from XML file data connections - that's easier for a sample (since you can't connect to my SharePoint or SQL) but hopefully will get the idea across.
I named my data sources SharePoint and SQL - I'm guessing that in general your SharePoint list will be a subset of the SQL list - but at any rate, preview this and see if it is what you are looking to do. Then you can check the filter on the list box data source. :)