UPDATED with the new YoVille Layout:
Simply go to your house (click it on the bottom)
once you click on that you will see this
Which ever house you choose will become your default home.
OLD WAY:
Ok since I had this issue and green had this issue I thought I would show you how to find your home on YoVille once you purchase it. See even though you buy a home on yoville it doesn't automatically make it your default home. You have to make it your default.
Up at the top where it says "Go Home" to the right of it is a arrow - click on it
Now you will see all of the houses you have purchased along with your original apt. As of now when you upgrade to a new house you can not sell your old house. You can't even give it away 
OK click on the house you would like to make your "Default" home. It will then take you to your home. At the bottom of the home you will see a box - at first it will only say "Your". Click on that box and you will get another pop-up box. Name your home and then choose it as your default.
Now just some advice when you move take EVERYTHING with you. Including your whole kitchen (counters and all) and all of your windows. You will need those in your new house. Now yes you can pop back over to your old house to get them later. All of your homes are available at all times for you to visit. Your friends only see your default home 
Are you sure you set your house as your default? I do notice that when I accept a gift from friends and it takes me to their house that it will take me to their empty apartment too. I think that Zynga has some bugs in the program for sure. If they click on your name below where is streams all of your yoville friends do they get taken to your house or your apartment?
I clicked on the "your home" at the bottom of the screen, but it didn't do anything...
Thanks for checking, makes me feel a little better. Thought it was something with my computer. Hope it gets fixed soon, my friends will be visiting an empty apartment! 
OK this is really annoying that you can't change homes but I guess they are working on it: CLICKY
Hey Everyone,
Our team is aware of the issue of not being able to set your home as your default or change the name of your home. The pop-up itself to make any of these changes is not appearing - making these changes not available in game. We are working hard to get this resolved and giving you these options again as soon as possible. Thanks for your patience while we fix this!
- Platinum Ninja
while in your home, over your progress on the top right side...You will see "home" or whatever you named your house, click on that to change to default home
Questions......I bought a house in Yoville and went out for a few new neighbors today. I think they are visiting me through the neighbor page or the apartment page. My house is the default but they can only get to my empty apartment?
Thanks!
Michelle