The Domelight is actually a custom light and should be attached to the CustomShaders->LightShader slot of an AreaLight. This is the easy setup. Better you set the HighSamples very high so you won’t have to touch that anymore and will be free to do all operations from the Domelight rollout without returning to the AreaLight parameters (HighSamples is not used by the Domelight, however looks like mental ray won’t obey the sample parameter you set in the Domelight if it is > of HighSamples. So set it huge and just forget about.
The Domelight itself has very few and easy parameters. Defaults should be OK in most cases.
Once you setup the Domelight, all you have to do is to attach an environment shader to the camera mental ray environment slot.
From the Domelight UI you can setup both an IBL and the new Sun&Sky. Eventually you can setup both and layer them up.
There’re also useful parameters to let you rotate (xyz) and pan (xy) your environment map. Zooming is used only for background images.
A basic scene with Sun&Sky :
