In Victoria II, there are no commands that can change the population of your nation however, there are events that can be started with commands that do basically the same thing.
Commands are typed into the console, to open the console press ` (grave) or ~ (tilde) on your keyboard. If neither of those work, see this help page.
To increase your nation's population, the "Men for The Colonies!" event is used (also known as the "Baby Boom" event). This will cause one province in your nation (random) to have a 1% increase in population. By spamming this event, you can increase the population of your nation pretty quickly. All you have to do is type the following into your console repeatedly: