This machine has an electronic thread tension control. You'll only be able to adjust the tension for the current stitch, until you turn off the machine or change your stitch selection. Press the Setting button and use the Up/Down arrows on the wheel to get the move the setting selection box on the Thread tension line, if needed. Use the Left/Right arrows on the wheel to increase or decrease the tension. Press OK (the center of the wheel) to save this tension. Test the tension on a scrap of your project and repeat the steps until the thread tension is appropriate for the project. Press Info when in the sewing mode to see the current thread tension.
The manual for the Pfaff Quilt Expression 4.0 (and a couple of other models in the Expression line) is available from Pfaff here: http://www.pfaff.com/SiteMedia/PFAFF/Products/Machines/Support-manuals/expression_2_3_4-manual-EN.pdf?ext=.pdf
. See page 2:16 (page 28 of the PDF) for the details. Pages 2:10-2:11 show the buttons for the Expression 4.0.
I hope this helps.