Restarted my Mac; that diid nothing. Next, I trashed com.camerabits.PhotoMechanic.plist & com.camerabits.PMNavigationAgent.plist thinking that might restore default settings. Now I can at least drag the vertical left bar but I only get a black window, not the navigator pane. I can drag a folder into the main window, but can't drag it into the navigator pane. I've looked through all the preference panes, but can't find anything similar to "restore defaults."
Finally discovered the solution after trying a bunch of random stuff: don't just click and drag on the left vertical bar. Instead, shift-click-drag. Must've changed since PM5 since this is an older post.