If you take the seatbox cover off and look again, you will see a large Bolt/Hex Head" offset on the driver side(assuming LHD) This is your dipstick........it's under there, just unscrew it.
There is a notch about 3/4 of the way down the dipstick, if it's wet with it screwed all the way in you are good to go. Otherwise add 90 weight.
I fill and maintain my fairy OD independent of the transfer case. I have another truck with a Roverdrive and it shares the 90 weight with the transfer case, this is where you are likely getting conflicting information between several different manufacturers.
------ Follow up post added March 17th, 2015 06:05 PM ------
Here is the manual.
This one appears to have a flat head screw for the dipstick, but I have never seen that before. I have had three Fairy's and they all had a hex head for the dipstick.
Anyway, hopefully this is helpful.
Land Rover FAQ - Repair & Maintenance - Series - Options - Fairey Overdrive Installation Manual Pg2
Navigate the pages on the left part of the screen.