This worked. Thanks!
You’re very welcome. Cheers!
Glad I could find my question answered here:)
I have the same problem. There are two solution which both do not work for me.
(1) I cannot install the modules, they are installed, greyed out in the dialogue window.
The OCR works, but I do not see the Mail PlugIN which I do not care about at the moments.
(2) I tried to drag the INBOX for Devonthink 3 into the sidebar, it looks as if I can place it there, but when I release the mouse button, the INBOX does not appear in the side bar.
I am working on Mojave, actual version.
The INBOX in the sidebar is very much needed!
The plugin has to be enabled in Apple Mail > Preferences > General > Manage Plugins…
The inbox folder (not database) can be added to the Finder’s sidebar, just drag & drop ~/Library/Application Support/DEVONthink 3/Inbox to it.
Thanks, this was helpful, the inbox is there 
I expected the Inbox to have this special “DEVONthink inbox” look, and never found it. Now that I took the screenshot, I saw this absolutely “NORMAL” inbox, and after opening it, it had the “DEVONthink inbox” look in the window top bar and – it is it!
Sorry for the inconvenience,
Maria
Thank you for your comments, all was done, and now it works, thanks to BLUEFROG.
No worries! 
Now it’s my turn. I upgraded my systems to DT3 last night and the finder INBOX is not working. I did the Install Add-Ons as described here. Is there something else that I need to do?
Do you have DEVONthink 2 installed?
Yes… Should I delete that?
Yes, definitely. At a minimum, delete the DEVONthink 2 application from the Applications folder. And rebooting the machine wouldn’t be a bad idea.
I deleted DT2 from the App folder and rebooted but no change in behavior
Control-click and remove it from the Finder’s sidebar then install it again from DEVONthink 3 > Install Add-Ons… and do not rename the sidebar alias (in case you did or wanted to).
I removed it from the sidebar. The Install Add Ons is grayed out for that option.
Post a screen capture of the Finder’s sidebar.
Never mind. Inbox reappeared in the Sidebar and it is working now… 
(It appeared at the bottom and I wasn’t looking there)
Much thanks…
You’re welcome
Sorry to rehash an old thread, just wanted to note that I came here via google because I also didn’t realize that “Inbox” in my favorites referred to my Devonthink Inbox.
I’m not sure if it’s worth changing, just thought devs would like to note the stumbling block.