MyBB Community Forums

How do I add a reputation bar in posts like in this forum?

It's not like that.
It's next to the 'Search' icon.

If you didn't understand I will take a screenshot

^ do you mean that your forum has no reputation button in posts () ?
may be reputation is turned off in the settings ...

Admin panel --> configuration --> settings --> Reputation --> Allow Post Reputations

And how do I add in a new theme that doesn't haves?

Make sure {$post['button_rep']} is in the postbit or postbit_classic template.

There is in postbit_classic.

Are you even using the classic postbit? If not, you need to check the postbit template. Not the postbit_classic template.

Whats the diference beetwen them btw?

Classic postbit is the vertical view, whereas the normal postbit is the horizontal view.