Hello,

I have the Yellow Portable Jump Starter with Air Compressor,

The problem is I have charged it fully and it always stays at 13 on the meter
Which is in the green which is supposed to be good.

When my cars battery goes flat it doesn't charge the battery, doesn't do anything,
But it does have plenty of power in it so its not as if its flat.

I have a MK4 Fiesta what part of the engine should I connect the Black Negative to,
I believe it may be due to me not connecting the Black Negative to a good part of the engine.

Will it have enough power at 13 on the meter to charge a flat battery?

Thank You

First off, these jump boxes we call them, do not charge batteries. They "assist" the battery being jumped. These jump boxes nowadays have an "on" switch for safety reasons for after you've hooked up and hopefuly in correct polarity. Grounding points are anywhere on the engine with a good solid bite. Idealy where the ground cable connects to.
If you have to, use the negative post on the battery. Just be careful.
It's possible that your dead battery is so dead, that this jump box just isn't going to work.
Have an assistant turn the key while you hold and wiggle the cables if need to. Electrical can be funny like that sometimes.
I think it's time to get rid of the yellow, and plant down some green on a new battery In my opinion.