Hello all, I did some further testing with a computer running a KeyShot Pro version, and it indeed opens OBJ and STP files with ease. I'm really liking MoI and I'm looking forward to version 4. (While the subtool bit might not be of interest to you, I'm just mentioning it in case other ZBrush users browse the forum at some stage and have the same issue I had.) But when I turn on 'show polygroups', it neatly shows the different elements as I put them together in MoI, so then I just 'group split' all the polygroups which results in separate subtools, and thus allows me to assign materials as I wish once I send the model to KeyShot via the ZBrush Bridge. Note that when I open the MoI model in ZBrush, it shows up as a single subtool. It's a bit of a roundabout way of getting my MoI model into KeyShot, but it does work.

I did some more testing and digging and it turns out that the KeyShot version I have is 'KeyShot for ZBrush' and I can only open OBJ files if I open them in ZBrush first, and then send them to KeyShot via the 'ZBrush to KeyShot Bridge' (the regular, but more expensive, KeyShot version shouldn't have any issues opening OBJ files - my cheaper version is limited in what it can open).

Zbrush to keyshot without bridge