I've looked for a solid half hour and can't find the keybind. I checked the mod options config, and the in game keybind setter and can't find it anywhere.

I checked the config file and it said "PERIOD" was the keybind. So i've now tried that but it doesn't work

The config name is overlaySlimeChunks in the Renderers tab. If PERIOD doesn't work, try using something else like H+S for example. Also make sure the main rendering is enabled.

The slime chunk overlay is under the "Renderers" tab. You have to set the keybind yourself by clicking the button next to the true/false button.

You also need to make sure the mainRenderingToggle is on to see any of the renderers in MiniHUD.