I need help opening secondary cargo doors!

If your aircraft model doesn't support this function, you won't be able to do that. If it does, press Shift+E and then '2' after 0.2 0.3 seconds. The same is with pushback left or or right, you press shift+P and then 1 for left or 2 for right just moments after letting shift and P or E go. If you don't make it, try to change keyboard assignment. Let us say for ctrl+2 or whatever.

Need it to be more simple.

Depending on the aircraft that you are using, the cargo door may not open. Hit [Shift+E], slight pause, then hit [2]. If that does not work, do it again except substitute 3, 4, 5 for the place you hit 2. If that doesn't work, the door is not made to open. If it's a downloaded or payware aircraft, check the read me file/manual for special instructions. 🙂