Note that this tool is only available inside a GeoNadir project.
Hover over an orthomosaic, DSM, or DTM and you will see the transparency slider appear. Make sure that your layer of interest is 'turned on' so that the effect is visible.
Move the slider and observe the effect on your map! By default the slider will be all the way to the right, which means that your layer should be clearly visible.
As you move the slider to the left, it will become transparent.
Note that if you change the transparency of a layer that is below another, you will not see the effect.