As far as I can tell, Accelerator bonuses are given automatically when the ticket is booked via EK website.
However, if you book via a TA (or online TA), then you must 'opt-in' to that month's Accelerator program. I have a booking made via a TA that showed up first with no bonus mileage. However, after ticketing the reservation via TA, I discovered the sign-up for Accelerator and did so. Seeing as I did so in the same month as ticketing, MMB now shows the bonus mileage also... and I can only assume it'll be received, as I have yet to fly it.