How to open the options window?

Per post title, how do you open the options window like in the screenshots?

Ta.

Look for the icon highlighted in the image.

Thanks for the reply, is it possible that there is a limit to how many icons can be on the bottom bar, or maybe they are positional requirements that conflict with another mod I have? I have several options there already from other mods, but not an icon like the one highlighted.

I recommend these mods:

These work very well together and you can customize the entire look of the UI.

And if you want a neat research tree, here's this mod:

Thanks for the tip.