I am sure you must have found the answer to this question. If you have, please feel free to tell us your findings. If not, perhaps, this is the reason.
In VOF approach, there are primary and secondary phases. Once you assign them, you will have to explicitly select each 'Phase' from the boundary conditions tab to denote the volume fraction. For boundary conditions such as velocity inlet, pressure outlet, walls etc., the default 'Phase' is set to mixture. Here you will not be able to prescribe a volume fraction and the multiphase tab is disabled. This tab will also be disabled when the 'Phase' is changed to primary phase. Once you change the 'Phase' to reflect the secondary, you will be able to prescribe a volume fraction.
I hope this is what you were looking for.