I have spent about 4 hours with 3D and have not been able to shot down a single plane. I have set the reality panel to easiest and I am using the E key to have enemy planes centered in the middle of my screen. It was easier to shoot down planes in RBI. Does any1 have any tips that could help me.

Are you playing the stock game? With mods it is harder to bring down a plane. Other than that there is not much you can do but practice in Fly Now until you get the hang of it.

I have only played the game in "Fly Now" mode. Can you tell me what is the "Stock Game".

By stock, he means just the base game with no user-created content.

There are many "patches" (UOP - UnOfficial Patch)available for the game which upgrade every aspect of RB3D.
There are also utilities such as "ReLoad", CM11 and FCJCP, and editors programs.
"Stock" means you are using none of these, your planes have no engines and paper thin wheels & wings, etc.
Use F1 view to shoot at planes. Use "E" (uses F2 view)to locate an enemy then switch to F1 when he's in front of you. Switch back to F2 if he gets out of sight and try again. Work one victim at a time, you can't afford to be spreading your bullets around on multiple targets.
I'm supposing you have the crosshairs enabled. It's big help in F1 view.
Most RL WWI pilots never shot down a plane and most who did picked on recon aircraft.
Good Luck!