If YOU can wither, can monsters, animals and plants wither too?
Not yet. I'm not completely sure if I want to add thirst and energy to animals, monsters, or plants yet. Plants, not being entities, cannot have the wither potion effect added to them. However, they will "wither" and die in the future.
I need help the version creator doesn't work. When I press install it doesn't do anything.
Have you checked your Minecraft install folder (%appdata%\.minecraft on Windows)? It makes a new folder in the versions folder, after which you open the zip with the patch files (these are two separate downloads) and drag the files into the jar. However, I highly suggest that you use MCPatcher instead, as it is much easier to work with than my helper and will install the mod fully, whereas mine requires a bit more work.