Plastic Screw on top of lower spray arm

[ladyrepair]

Whirlpool dishwasher hums on wash.  I want to remove inside lower housing unit to check for foreign objects.  There is a plastic thumb screw on top.  I keep turning it, but it doesn't seem to be doing anything but turning.

How do I get it off?

Thank you.

Model DUL300XTLT3

[Repair-man]

You must follow the same directions we do:

REMOVE/REINSTALL WASH PUMP ASSEMBLY
1. Remove any remaining water in the dishwasher. If possible, the CANCEL/DRAIN button may be used.
2. Unplug dishwasher or disconnect power.
3. Open dishwasher door and remove lower rack.
4. Pull upper rack partially forward and detach the water delivery system from the tub anchor points (2 clips on stainless models; 2 screws on plastic models). Removal of the upper rack may ease removal of the water delivery system, but is not necessary.
5. At rear of pump, grasp the water delivery system and rotate clockwise while lifting upward. When rotation stops, lift the lower spray arm until it is clear of the pump and pull forward. The spray arm assembly and water delivery system should now be free for removal.
6. If only inspecting/servicing the chopper system, repairing the filter assembly, or removing foreign objects, skip to “Chopper/Filter Service” section at right.
7. To separate the pump from the tub assembly, close the dishwasher door and remove the lower access panel. (Additional clearance for working under the unit can be gained by removing the door panel.)
8. Place a shallow pan under the drain pump area tocatch any water remaining in the unit.
9. Disconnect the drain loop from the pump byremoving the clamp and pulling the hose forward. If equipped, also remove the pressure sensor by removing the screw and pulling the part forward.
10. Remove the drain pump by depressing the locking tab and rotating the pump  counterclockwise (as viewed from the right side of unit).
11. Slide shallow pan out and discard water caught.
12. Disconnect the thermistor and motor electrical connections.
13. Remove 3 pump-retaining tabs.
14. Press upward on the pump to dislodge. Push the assembly toward the rear of dishwasher and rotate the front upward until the drain port is above the tub opening. Rest the pump in a stable position.

Service Data Sheet

[Mr. Fix-It]

Hold the spray arm with one hand and turn the thumbscrew with the other . Sometimes you have to use a pair of plyers.

[ladyrepair]

This what I've been doing, but the screw keeps turning and not coming off. 

How long is the screw?