Bingo Pop Rooms on the main path are unlocked by reaching a certain player level - you can see the level you'll need to reach on each "locked" Room icon.
 
Your current level is the green number beneath your profile picture in the top left of the Room Selection screen. Increase your player level by earning XP (Experience Points)! 
 
You can view your current XP, followed by the amount you'll need to reach the next level, by tapping your profile picture.