Okay, I've been on the D-50, and:

Press L-Tone Edit, this will take you to an edit screen:

(T-Name) (Common) (Part-1) (Part-2) (T-Copy)
Which correspond to:
Key Split Tone L-Tone U-Tone
Mode Point Balance Edit Edit

NB: The Partial select buttons can be used here to select which partial you wish to edit:
1 Lower 2 1 Upper 2 (first 4 bank select buttons).

Pressing "Tone Balance" for 'Part-1', this will take you to:
(Pitch) (Form) ( TVF ) ( TVA ) * Init *
Pressing "Tone Balance" again will take you to the TVF (Time variant filter) menu:
Now press the "Patch Edit" / __\ key to take you to the next screen:
Dpth Velo DKF TFK
Velo (Split Point key) is the velocity sensitivity, and ranges from 0-100, you can use the joystick, or inc / dec keys to change the value.

Going back to the 2nd edit screen, if you press L-Tone Edit key (TVA) it takes you straight the Time Variant Amplifier menu, and Velo is the 2nd parameter. It ranges for -50 to +50. "0" = no velocity.