The list box should be bound to a field in your main data source. Then use the secondary connection as the source for the list. I am guessing that your list box is bound directly to a field in the data connection which is not how you want to set it up.
1 would be to drag a repeating table on to verify what is coming in via your secondary data connection.
2 add a list control to your form which should automatically create a new field.
3 go into the properties of the list control and select to use values from an external data source and select your data connection from the dropdown.
Play around with it.
Hope this helps.