Hi Enigma,
See if this work, SharePoint only passes the ID of a lookup, not the value.
1. Add a data connection to the other SharePoint list (where Display name is located)
2. Change the textbox to a calculated field (where you will output the Display name)
3. Right click on the calculated field, control properties, then fx
4. In the formula, -> set a fields' value -> select the new data connection you created earlier -> choose the field for the display name -> filter by ID.
5. Filter by ID of the corresponding column (the drop down)
cheers!