How do you actually toggle peace mode while using this? I saw a post saying there should be a button in the top left but there is no button.

Sorry for the late reply, I'm not getting notifications for these discussions.
The mod should add a small in-game button on the top left of your screen.

If there's not, please make sure the mod is enabled.

what does the cross mark mean, mod disabled or enabled?

The button show the current "Peaceful" state of biters.

If the button is completely green, the biters are peaceful.
If the button has the red cross over it, the biters are -not- peaceful.

I've updated the mod for 2.1, and rewritten it so the button is now moved to the shortcut bar, with the option to assign a keyboard shortcut.
The mod is now fully compatible with Space Age, and works on Gleba and Vulcanus as well.

This means that the previous information is now outdated.
When the shortcut button is "active" (orange), peaceful mode is enabled. You'll also see a notification of the toggle, in chat.