Any chance you can attach a packaged workbook showing what you have so far?
Hi Genevieve - if you want to use the Tableau generated lat/lon values, you'll have to export those locations and join the generated lat/lon back into your workbook. You can't currently reference those directly in calculations. If you need to use MakeLine for an origin and destination that are BOTH Tableau geocoding locations (e.g., the IATA airport codes) you'll do two joins to get the airport lat/lon values into your data.
To export the generated lat/lon values, map your airports, select them all, and view the data. You'll see columns for the generated lat and lon values. Export the table. Then join this table back in so you can use the generated lat/lon values in the data source instead of trying to use the Tableau generated lat/lon values.
As an example of how to export the data, this is what I see when I select some data on my map...if I want the lat/lon generated I'd just use export all button. Make sure you're getting a field that will be a good unique id to join the exported data back in (e.g., the airport codes)
Also, as Ken noted above, if you can provide a sample workbook (even just a table with a sample of the data) we can probably help mock up a little demo of how this would work to get MakeLine functioning for your spidermap.