[Graham Harvey]

Can you somehow open an MXI into Photoshop and adjust lighting by adjusting the opacity of layers?

[T0M0]

You should open MXI file in Photoshop in 32bit depth, set layers to Linear Dodge (Add) and adjust them with exposure.

more info - Maxwell Plugin Documention

[Graham Harvey]

My version of Photoshop (CC 2015) does not open MXI files. What version do you have!?

[T0M0]

2015 CC (2015.1.2)

Do you have latest Maxwell Photoshop 3.2.0.0 plugin installed ? You can download it via customer portal.