Here's what's printed in my WSM in the event it is different than yours. They had several updates over the years.
(12) Mode "K" (HST Pedal, Range Shift Lever, Cruise Control Lever, HST
Response Dial, Fine-adjustment Mode)
NOTE
***8226; When any of ECU, HST pedal sensor, cruise control lever position sensor, swashplate position sensor, range gear shift sensor, HST response dial or proportional valve (Forward, Reverse) is replaced, this adjustment is required.
***8226; Tractor Condition
Parking brake: Set the Apply the Parking Brake (To push the cruise control release switch)
Cruise Control Lever: Set the Neutral Position
HST Pedal: Set the Neutral Position
Range Gear Shift Lever: Set the Neutral Position (between L and M)
HST Response Dial: Set the Center Detente Position
1. While holding down the both switches at once, start the engine at idling speed.
2. Select the mode "K".
3. Hold down the display mode switch for more than 2 seconds.
4. "HST-ADJ" (b) is displayed and release the switch.
NOTE
***8226; When indication "HST-ADJ" does not change, the engine is not idling, or the parking brake is not used.
***8226; When "ERROR-xx" is displayed (k), refer to the below chart.
5. "PUSH-SW" is displayed (c): Hold down the displayed mode
switch for more than 2 seconds.
(Reference)
Error Display..Fine-adjustment
ERROR-71..... Range Gear Shift Sensor
ERROR-72..... HST Pedal Sensor
ERROR-73..... Swashplate Position Sensor
ERROR-74..... Cruise Control Lever Position Sensor
ERROR-75..... Accelerator Lever Sensor
ERROR-76..... HST Response Dial
(a) Mode Selection Display
(b) Information Display
(c) Setting Display-1
(d) Adjusting Display-1
(e) Setting Display-2
(f) Adjusting Display-2
(g) Setting Display-3
(h) Setting Display-4
(i) Adjusting Display-3
(j) Correct Preserving Display
(k) Error Display-1
(l) Error Display-2
(m) Error Display-3
****************End of manual*********
There's another page to the instructions, but it is beyond where you're having trouble.
Never tried it myself, so I can't comment further.
Good luck!