For this tutorial, i will use the Source Code of BuildBroken´s Wooden Rails in MC 1.7.10 and You can do the same with another mods without another mod dependencies (You cant use Mods like Buildcraft or IC2)
First of all, Do an Backup of your entire MCreator´s installation folder, then Make another backup in the desktop (In this instance i will call it ´´test´´), then delete all the workspace´s mods (Yeah, thats why the two backups), let it finish the task and close mcreator.
Download an source code of an mod (Again, its of BuildBroken´s Wooden Rails) and unzip its contents in the desktop
Then open the folder where you located the MCreator (The one you cleaned of mods) and go to forge/src/main/java/mod and you´ll see an folder called ´´Mcreator´´, do the same with the Source code you just downloaded (Instead of mod, it can be named com or net), open the folder and copy the inside (In this case its BuildBroken), paste inside forge/src/main/java/mod and NOT inside MCreator´s folder, close the file manager´s windows and open MCreator
Go to Code section and you´ll be able of editing the man code of the mod.
And, Thats all! i will finish Wooden Rails mod
Images coming soon