To access Dream World, you need to find blue mushrooms. If you don't know how to find them, you have to free Patchouli in the first dungeon. Talk to her for a few days and she'll tell you about those shrooms. You'll then be able to find them around the island, they're always in the same spots, so look around for a bit. They take something around two days to respawn so that's that. Finally, you have to use those blue shrooms on your 3 family members after midnight. Talk to them in the morning and they'll tell you about the dreams they had. You'll now have the Dreamworld unlocked. Just go to bed and select the option with the blue mushroom icon.
As said before, Cirno spawns in front of the monster girl base. If you don't know where it is or haven't built it yet and just want to find Cirno, press F10 to open the teleport menu and select To the Monster Base. However, you might not be able to access her scenes without the monster girls' base built. If you don't have it yet, you first have to catch the fish girl that roams the grassy plain - the one where the big pink tree and Velvet's tent are, so look thoroughly all the way. The option to build the base will be unlocked, close to your own tent. You may now capture the fish girl, as well as two other monsters girls that will spawn in the following days at the same spot (and also the two dungeon bosses, but if you've already finished the dungeons, you can't get the bosses anymore, at least not without starting a new game and building the base before that).