Then the way you'd get the additional fillet you're showing with the yellow line would be to select this entire chain of edges like this:



Then the filleter will be able to put in a corner juncture at the ending point:




But again this requires a cleaner model structure in order to work, there isn't much chance that the filleter is going to be able to do anything on the sort of squiggly network surface that you've got in the corner area right now. Probably the best bet for putting in something on your current model without rebuilding it to a cleaner structure would be to slice away some space and then use Construct > Blend to put in some smooth pieces, filleting is not going to be likely to work on spots bounded by squiggly little pieces.

EDIT: Didn't see Andrei's post saying the same thing...

- Michael