I see that this is an old topic, but I have only recently renewed my Object Desktop subscription and found that Direct Folders won't come up. For the last two years I have been using Direct Folders and can confirm that - yes - double clicking on an open spot on the desktop will indeed open the DF-menu.
In addition to that functionality failing to work, though, I also cannot get the DF-menu to open when I double click inside an active Windows explorer window. I have added exceptions to both DF and WB and cannot determine the problem.
If any of you have a thought about what I could try, let me know. Everything's been stellar right up until I installed the Object Desktop suite.