This add-on adds an "Add-ons" option to the list of available engines on the Sync preferences panel of SeaMonkey 2.8. SeaMonkey 2.8 already includes the back-end support required to sync add-ons (like Firefox 11); only the configuration front-end is missing.
SeaMonkey 2.9 and later versions will include the Add-ons option out of the box (and additionally on the Sync setup wizard), and earlier versions (like 2.7) lack the back-end support for this feature. Thus this add-on will really only work with SeaMonkey 2.8. If you try to run it with any other version of SeaMonkey it will not do any harm, though, but just disable itself and print a message on the Error Console.