It won't do it 'instantly'. Turn on the settings to pull sims from your gallery when the game makes new sims, and over time it will fully populate, but it's something that happens over the course of many sim days, not instantly. This setting is under MC Population->Populating Settings->Import Tray Settings
Adjust the settings for MC Population->Moving Settings->Open Houses to keep from having 'every actual home filled'. If you want it so that NO homeless sims get moved in automatically, Disable Allow Homeless Move-In instead.
I started a brand new save after the Import Tray option was added to MCCC last week and it took maybe a week or so of in game time before there was a really healthy number of households in the game, but it generated enough right away that my Sims weren't alone in the world. Now, 3 weeks in, the worlds have about half the houses populated (and I have it set to allow all except 1 house to get used) and there are another 84 households "currently not in world". What's really nice about those 84 households is that they are diverse, dress well...and don't have either ridiculous or repetitive names.