So I went into the mod file and I didn't have a custom rewards folder I check everywhere and didn't have one.

Are you sure you've checked the config\ChanceCubes folder and the CustomRewards folder does not exist?

yeah I just rechecked and all that was in the folder were class files

You need to install the mod first (put the jar in the mods folder with Forge installed), then it'll generate the CustomRewards folder.
The jar file itself is just the compiled source code, that's why there are class files.

The folder will be in the config folder along with all of the other mods' configs

^

Ok I was looking in the config folder in the mod itself