I ran into this exact problem earlier this afternoon as well. The .DAT in question is ROM/119/51.DAT. From what I can tell, this .DAT concerns itself only with the in-game font and interface menu.
Chances are, you installed a previous version of this .DAT, where the UI has not accommodated for the size changes of the in-game menus in this latest update. Its kind of difficult to explain without seeing it in person... the buttons are still there and function properly in game, they just wont be labeled correctly and the last button on the list simply wont exist. For example, while in your mog house, the Delivery Box will open your Mog Sack, and selecting "Change Job" will actually open your Delivery Box.
You'll need the up to date version of ROM/119/51.DAT, either by using the "Check Files" option or by using a current UI installer, patched for the 12/6/10 update.
Luckily, XI View has been updated to accommodate this, and you can download the current build (V1.04)
at the Windower website. If you dont like all the other additions and features of XI View, you can simply install the single ROM/119/51.DAT instead of the entire XI View package.