First, create a folder within PlainText’s directory in your Dropbox. [1. I’m still waiting for the day when these apps become less picky about their location.] Next, go to File > Sync in Scrivener and select With External Folder. A new slip appears. Navigate to the folder you created on Dropbox. Be sure to select Format External Drafts as plain text. Finally, hit sync and you’re done. Now updates made with Scrivener will appear in PlainText and vice versa.
There are some caveats to remember. First, sync isn’t automatic. You must manually initiate a sync via Scrivener’s file menu on your Mac. Second, you can’t change page titles in PlainText; doing so will mess things up in a big way. Finally, only your copy is synchronized, meaning no reference material on the iPad. Still, this setup works when you have the time and inclination to write and only your iPad.