I’m currently running a incomprehensible simulation on a car, but when I added two cartesian boxes for refinement, the options to select then doesn’t popup. Any help would be appreciated!
Hey @silverracing47 ,
If you look below the ‘Refinement regions’ option in the ‘Volume custom sizing’ refinement, you should see toggles (highlighted in below image) for the Geometry Primitives you created.
You seem to have figured that out and seem to have successfully run the simulations. Let us know if you need more support here. 
Best,
Satvik
@silverracing47 as an addition to Satvik’s comment, please note that geometry primitives created on the geometry level don’t carry over to the meshing one - you’d need to recreate them under Mesh \rightarrow Geometry primitives