I have a 6 numeric digit booking reference which I got from the WDTC (UK) when I booked. However, this wasn't recognised when I tried to set up My Disney Experience on the WDW website, so I called Disney and received an 8 digit (999AAAAA format) which did work. However, since I received mine (a good few months ago now), Disney may have changed the system to recognise the UK booking refs.
I was able to link my account with the 6 digit number through MDE in December (we booked in September) after failing to get hold of an 8 digit number on multiple occasions.