On the NUM Pad press 4 then 6 to see the ailerons move to bank and turn the aircraft left and right. Start the Resort Supply mission with the Beechcraft Baron in tail view. This is necessary as cargo changes from one port to another and fuel is used up or replenished. With a ship you move ballast around to maintain a bow up condition and a level deck as well as port/starboard balance at zero degrees of inclination. You are trimming the aircraft to maintain level flight. The word 'trim' explains precisely what the 'trim' function does using the trim tabs on the tailplane. You can set the appropriate trim and then let go of your controls, like in real airplanes. Trim is more useful to joystick and yoke users because you don't have to hold onto the controls to maintain straight and level flight or a preferred climb/descent rate. As a mouse/keyboard user, you probably won't even need trim because the controls are too fine to require trim.
Trim should not be used as a primary climb/descend control. You are supposed to use primary flight controls first (keypad or mouse yoke) for primary controls and only use trim to fine tune your controls. You can mouse over that wheel, or any trim up/down indicator if using a different aircraft, to see how much trim you have in +/- numbers. It is in all C172 pre-takeoff checklists.
You want to move the trip to match the line before you take off. The C172 has a line that indicated whether the trim is above or below the TO line, and by how much. TO means the trim setting that is used for takeoff. If your aircraft has a trim wheel like the default C172, you can mouse over it and use the wheel up/down. If you are also using the num pad for flight controls then trim should work. You need to turn off Num Lock to be able to use the keys for elevator trim.