I was just having a problem with this as well and figured it out. If you right click on a tabbed dock and go to add>tabs>folder tab> you will get a window asking for you to specify a folder, this is where i got hung up. the problem for me is object dock is asking for me to specify a folder but it seems to be looking for a file and not a folder and it wont recognize any folders. just click on any file inside the folder you want to have tabbed and it will then create the new explorer tab.