Search on the internet for CnC Generals Zero Hour Mods/trainers, most of them should either have instructions included with the files or on the website. If not, contact the file's creator and ask for help.
A good mod that I'm using is called: Command And Conquer Generals Shockwave. When you find the website on the web, download it, but if you try it then it won't work. You also have to get a patch. Search: "Command and Conquer Generals Shockwave Patch 1.04" when you find it, download it, and- have fun!
There is a much easier way to play as Boss General Leang without having to download a mod. Use a program like FinalBig to extract INIZH.BIG's contents in Zero Hour's main directory. Open "ChallengeMode.ini" in the INI folder, scroll down to the block labeled "GeneralPersona9" and change "StartsEnabled = no" to "StartsEnabled = yes". That's all you need to do. If you want to play as General Leang in the Challenge mode, you would need to download a mod since there is no AI support for her in Command and Conquer Generals Zero Hour's maps (except for her own map). For mods, try Pro:Gen, Shockwave, and others at http://commandandconquer.filefront.com or http://www.moddb.com
Command blocks are only obtained by creative or mods. You must be in creative mode in order to successfully type a command into the command block, otherwise you will be prompted to be in creative mode.
Without mods, you need to make your world creative mode, not survival.
There are single player mods that enable this as a command. But most are for multiplayer if you are searching the plugin: WorldEdit is the most chosen
C&C4 was pretty depressing indeed. But the C&C universe is alive and well. With loyal fans coming up with creative mods and fan fare. I can't guarantee an official C&C5 (that would be sweet![if better than 4]) but there is still lots of C&C out there. Heck, RA is still going well, maybe there will be a RA4. :)
Certain downloads are only for certain computers, perhaps Single Player Command Mods don't work well with Macs
mods, mods, mods
moddb.com is good for mods. It has mount and blade mods and mods for virtually every other game.
you can get minecraft mods and textures off the minecraft mods website.
you can easily get mods from planetminecraft
The Mods was created in 1964.
look what ever game you want and add the word mods to the end on google. e.g oblivion mods, GTA mods