To see who give you Reputation, just follow click at UserCP link in the navbar.
In the buttom it will show you how much points you have, how give you reputation, when did you get reputation from him etc..
And if you want to add Reputation to any member just follow image:
1. Click at the Reputation box, near Online/Offline status icon.
2. Then click what reputation you want to add him (Postitive or Negative)
3. You can add any comment in the blank field.
4. Click at the "Add to Reputation" button, to finish.