Select the Chroma Key color by clicking on it in the preview image. Then, using the preview image as a reference, adjust the Background and Foreground sliders until the desired transparency is achieved. Note you can click different positions on the timeline to see how the video will look. Click OK to apply changes.
If you would like to make changes later, first select the overlay in the list on the Overlay tab, then click the Change button next to the Use Chroma Key checkbox.