Cargo Door

Shift+E.
What?
Go to control panel/system/hardware tab/device manager/display adapters/click on the + sign.

More to the point... IS there a cargo door???

Are there any addons for, cargo trucks and stuff, so you can see them loading cargo into the plane. Shift+E only opens the main exit, is there actually a cargo entrance?

Usually, you have to press Shift+E, quickly followed by 2, 3, or 4. These open secondary doors. On some add-on planes, you need to assign keys to the tailhook and wing fold to activate cargo exits. (or airstairs, wing fold, maintainance hatches, etc.)

I don't even know what aircraft your talking about.
Check out the read-me that came with it to see if it has one and what key he suggests to use.

See if this is what you're looking for:

Model features: Dynamic flexing wings that respond to turbulence. Fully Animated control surfaces, pushback truck with animated ground crew, opening doors & cargodoors,

➡ http://library.avsim.net/esearch.php?DLID=&Name=&FileName=os738w_saa.zip&Author=&CatID=root

If you want more toys to experiment with go here

➡ http://f1.aaa.livedoor.jp/~rjnnhama/craft.htm

BTW if you're not happy with the 737, Posky have other a/c with pushback trucks.