Minecraft Mods not working, why?

So, I've downloaded forge and made sure 'Install Client' was ticked. I downloaded a zip file and put it into the mods folder, but when I open Minecraft, I'm able to see the 'Mods' button in the menu where you can chose 'Single Player' and 'Multi Player' and when I click onto 'Mods' only the three initial mods are active. I've followed the 'Install Guide' while doing this, and it isn't a complicated install one like 'Single Player Commands'. I'm so confused about why it isn't working. The person who helps me to make it work will get 5 stars!

There's a few things you need to check:
1: Is the mod compatible with Forge?
2: Are you using the correct version of Forge for that mod?
The site for the mod should tell you what version of Forge and minecraft is needed for it to work.

If you can link the mod and tell me what version of forge you're using then I can test it myself.

1. Make sure if the mod needs Forge or ModLoader!
2. When opening minecraft launcher in bottom-left there's your username:
2.1.Click on it and select Forge…
3. Try now