The transition area is in the middle of the main window and looks like this:
Transition Buttons
There are three Transition buttons. In the image above, the transitions "Cut", "Smooth" and "Bowstring" are configured for the three buttons.
In the screen shot above, the transition "Smooth" is the currently selected transition (the one that will be used).
Changing the selected transition button
If you single-click on the "Bowstring" button, then it will become highlighted and "Bowstring" will be the current Transition, as shown here:
As you can see, you have 3 transitions at the ready at all times.
NOTE: You can also select one of the three buttons by using Command-1, Command-2, and Command-3, respectively.
Changing the transition for a transition button
If you click and drag on one of the three transition buttons, you can select a different transition for the button.
Here we've selected "Swoop" instead of "Cut":
Duration Button
The duration of the transition is controlled using this button: 
This button controls the duration of the transition (except for the "Cut" transition which is immediate).
If you select "Slow" from the popup, you'll notice that the icon contains a small Turtle, as shown here: 
Go Button
The Go button (or Command-G) will take any changes you have made and make them Live using the selected transition and duration.
However, the Go button only becomes important when you have turned AutoLive off. This is because when AutoLive is on, every time you click on a shot, the Go action automatically executed for you. See Usage of AutoLive for more information on using AutoLive.