Your Apple TV has two options you can either.
1.To watch Movies or TV's you purchased from the iTunes store select Movies or TV shows in the Apple TV's main menu.
2. To directly stream content such as dubbed movies or tv shows from your iMac you can select Computers in the Apple TV main menu.
3. The only way you can view folders is by selecting the Apple TV icon at the top dock of OSX, and selecting the Apple TV from the drop down menu. This allows you to mirror your desktop to the Apple TV.
4. As you cannot physically download apps on the Apple TV, the only way you can use apps on the Apple TV is through the Airplay function this allows you to mirror what app you are using on your iOS device to the TV. You can also use airplay to stream movies, tv shows and pictures that you are viewing on your iOS device. To use airplay there is TV icon on the toolbar menu.
Some important Tidbits
1. Dubbed movies must be in M4V i do not have a blu ray burner so to convert my DVDs I use a program called Ripit or you can download a freeware program called Handbrake.
2. The Apple TV must be connected to the internet whether wirelessly or physically via a ethernet connection.
Hope this helps.