I've been playing animal crossing for quite some time now. I really want to unlock the dream suite, but I haven't found Isabelle asleep. I've been mayor for more than 7 days, and I've connected to the internet.
As per the wiki:
The Dream Suite (also known as the Dream Mansion) is a building that is introduced in Animal Crossing: New Leaf. It is run and operated by Luna, who also makes her debut in the game. It must first be built, which can be achieved by talking to Isabelle when she is first seen sleeping at her desk in the Town Hall. She will make the suggestion shortly afterwards as a project.
So just visit the Town Hall until you catch her sleeping, after waking her up she will suggest the project and you will be able to access the Dream Suite.
If you go into Isabelle's wiki page you will also find:
She is seen to sleep in Town Hall, and when the player goes up to the front of the counter, she wakes up with a surprised look on her, and then acts embarrassed.
If I recall correctly she is most likely asleep early in the morning there or late at night.
The internet connection is not important immediately.