Under "Population details" in the Senate screen, it says I can boost a population using Luxury resources.

But I can't click on anything in that screen. How do I do this?

in order to be able to boost a pop migration you need to discover the luxury rescource assigne to this pop.

Every population has different favourite luxury resource. If you posses this resource, you can spend it here (button next to the race will become available) and in exchange choosen race will be born more often.

Thanks. I also saw this said as well on Steam.

It seems like I can have 6 different pop. types and NONE of the right resources.

Oh well..