Hi Mike - looking at your screenshot it appears that the revolve object and the other smaller one at the top are totally separate objects?

In order to fillet them you're going to need to boolean them together so that they are combined into one object which has shared edges at the place where they cross.

If you just have 2 totally separate individual objects they won't fillet with one another except for the special case of having 2 objects that are single surfaces and not joined-together surfaces.

So from what I can see you should use boolean union to merge those objects together, then there will be an edge there which you can fillet.

If you could post the 3DM model file it would still help me to more fully understand what is going on there, for example what exactly is the curve that you have highlighted in yellow there, is it an edge that is part of one of those solids, or is it possibly something like an independent curve object created from generating intersection curves between those objects or projected on to there?

There are a lot of details that are difficult to see only from a screenshot, so while a screenshot definitely helps more than just a text description, a 3DM model file also included also helps out a lot to figure things out.

- Michael