Did you buy your copy of OmniFocus from Omni, or the Mac App Store? If the latter, the domain name is slightly different. You can read about it here:
http://forums.omnigroup.com/showthread.php?t=21496
[tl;dr] use com.omnigroup.OmniFocus.MacAppStore if you bought from the Mac App Store
[tt;dw] in the Terminal, do
Code:
defaults read com.omnigroup.OmniFocus | grep -i sync
(use the com.omnigroup.OmniFocus.MacAppStore domain if appropriate)
and check that the desired value is set properly. You do need to quit OmniFocus before running the command to set it, or it will be overwritten.