Click on the join between two clips on the media timeline. Alternatively, move the cursor to the "Add Dissolve or Cross Fade" button, symbolized by a downwards and right pointing arrow located on the toolbar above the media timeline.
Click on the "Quick Transition" button on the toolbar above the media timeline to bring up the quick transition options.
Set the duration of the transition in the "Duration" box before setting the "Alignment" option to "Centered on Cut."
Click the "Tick" symbol to apply the quick transition.
Click the "E" button in the bottom right-hand corner of the window to open the render window. Click "Include Yellow Slices." Avid Liquid will render your transitions so that they will appear on your completed clip.