Why am I not surprised it's not in the manual? Most of the useful information isn't. At least with the WM240 & WM250 anyway. Both were several iterations out of date! OK for those familiar with the fine art of metal turning I guess, but what about the learners?
If you can't get the plug undone you can fill via one of the screw holes that hold the thread dial indicator to the carriage, one nearest the front being best option.