Malaysia airline reference numbers are five characters while British airways is six. Would like to add the MY details to my executive club account so I can get miles and also the executive club number is on ticket to allow access to the Oneworld lounge. Keep getting error message that I need six reference characters.
Your problem is you're trying to add them in the wrong place!
Very few One World partner bookings will show up in the BAEC interface. Generally only ones on 125- ticket stock (bookings made with British Airways themselves), and ones involving a BA leg will be available in the British Airways website. (Full gory details available on flyertalk if you really care...)
Normally, what you do is go to the Manage My Booking of the airline who's flying you, and tell them what your Frequent Flier details are.
So, for this case, go to the Malaysia Airlines website, and click the Manage link there. Enter your MH booking reference and surname, then from there enter your BA Frequent Flier details.
Short summary - tell the airline operating your flights what your frequent flier details are, not the airline who operates your frequent flyer program. (You'd only need to talk to your frequent flier airline if 2 weeks after flying the miles haven't credited)