In Bryce you have to choose the object, select the E (edit) button and once you get the smoothing box hit Ctrl+D (not sure what it is on a Mac) and you will get the export ability, scroll down, choose .obj and you're set. I regularly use Bryce for conversions. However you can only do that if it's an object that has been imported into bryce - doesn't work with booleans or other bryce-built objects.