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Control-click on the tool-bar and select "Customize toolbar..."

The third row has an icon labeled "Sync with iCal". That's the one you want to use to sync to iCal. Not the orange sync button.

You can also use Command-Option-S shortcut key or use the menu bar File > Sync with iCal.

I got stuck on this once upon a time too.

You might also tinker around with the preferences and make sure to map out which OmniFocus contexts will go to which calendar in iCal.

There is also a checkbox in preferences to sync blocked actions as well....