Can't Assign Midi Controls With Plugsound Pro Options

[grant610]

I use DP 5. I want to control the volume of a great organ sound on Plugsound Pro with my midi keyboard. I tried control clicking on the volume knob on Plugsound Pro. A window opens up. It says midi learn: CC 1. I clicked that box that says midi learn. I tried moving the volume fader on my Roland RD-700 keyboard. And I get no results!!! Any advice??