Autopilot Altitude setting for FMC

Hello

I regularly fly the B1900D in FSX and am about to embark on making a tour of the Uk video series on Youtube, I know how to set the autopilot using the knob to set the altitude in the cockpit but this seems to overide any alitiude setting I have in the FMC. Can anyone tell me what button I should be pressing so the autopilot goes to the FMC altitude?

Have a cheeky 2nd question too, Is it easy to install the B1900D into P3D V3.3? i am hoping that one day Carenado makes an offical installer  but have been waiting for this to happen and I really want to try it out. Trouble is I don't want to harm my FSX install

kind regards

Daz