Hello. I am new to rtabmap as well. I thought I would revive this thread since in the newest version changing these settings does not seem to activate the "export octomap" option. Are there any new settings that must be changed now?
I retested the options above and it does work. Local occupancy grid should be created in 3D (see Local Occupancy Grid panel) from depth image (or 3D point clouds if you have a 3D lidar). In Grid Map Assembling panel, Octomap box should be checked to construct the OctoMap, then the option in File menu should be enabled when the OctoMap is shown in 3D Map view.