Once you researched the civics in question (slavery for pop-rushing, universal suffrage for money-rushing) and applied them to your government, enter any city. On the city screen, between the minimap and and the area which shows the things you can build, there are some buttons, mostly for gearing the city towards science, production etc. There are also two buttons for rushing, directly under the "Draft" button. A mouse-over tells you what they are for and what will be the cost of the action. It's actually quite elegant once you get used to it, but I agree that it's not really obvious.
