When I tried to add a existing local sync store which is under Dropbox directory, it looks like all the folders are grey out and not able to add any of them.
I’m using 2.8.4 and OS X 10.10.3
A local syncStore should not be in a Dropbox folder, just as a DEVONthink database should not be.
And why not? I’ve been using Local Sync Store in a dropbox folder for years with no issues whatsoever. It continued to function flawlessly until today, when I had to reinstall DTPO after reformatting and now find myself unable to browse to dropbox locations. Completely annoying and frustrating.
The not being able to browse Dropbox locations sounds like a separate issue. If the ~/Application Support/DEVONthink Pro 2/Sync folder isn’t intact, you wouldn’t have an Sync Locations in Preferences > Sync.
Because it’s not made for this purpose. You can also consume small amounts of mercury for years, but it ultimately won’t end well.