You can access available transitions in the Transitions panel (click Transitions on the Action bar).
Quick view: All the available transitions appear as thumbnails in the Transitions panel. The Quick view contains a subset of the transitions available in the Expert view.
Expert view: The transitions are organized into categories. You can narrow a search for transitions by choosing a transition type, such as Dissolve, from the Category menu. You can also search for a transition by typing its name in the search box. The Expert view has more transitions compared to the Quick view.
Video transitions have animated thumbnail previews that show how they affect clips. Select a transition to set its thumbnail in motion. You can preview an animated thumbnail transition in the Transition panel without having to apply it to a clip.