Applicable versions and platforms
The Insync Finder extension was introduced in version 1.3.3. It handles the file icon overlays for OS X 10.10 (Yosemite) and later, as well as the context menu for OS X 10.11 (El Capitan) and later.
(Note that file icon overlays and the right-click context menu are available on other OS X versions, but are implemented using a different mechanism than the one described here.)
The "Add to Insync" context menu is not implemented by this extension, and so is not currently available on El Capitan. (fixed in 1.3.5)
Checking the status of the extension
The Insync Finder extension (as well as other third-party extensions) can be managed by going to System Preferences > Extensions.
Due to the way that Finder Sync extensions work, only one extension can be active for a given folder, even if multiple extensions are enabled.
For instance, the Dropbox extension watches the
Downloads folder. If the Insync folder is at
Documents/Insync and both the Insync and Dropbox extensions are active, only one of them will "win out", depending on which one was launched first.
To resolve the conflict, one of the two conflicting extensions must be disabled.