In both Firefox and Chrome in their current releases, Adblock Plus puts a button on the main toolbar to the right of the address box. It looks like a small red octagon with ABP in the middle for both, but on Firefox there is a small dropdown arrow on the right side of the button.
In Chrome, if you click on it you get something that appears to be a menu, but these days I doubt it is a menu, but at the bottom of whatever object it is there is a button or link for Options. In Firefox, if the button is activated a dropdown menu appears with Options as the last menu item. A button to contribute to Adblock Plus appears below that Options item.
