Where to find the serial number of my MPD218 for registration?

So a few months ago I bought the Akai MPD218, and while downloading Ableton I didn't have any problem, but now that I'm trying to download the other software (BigBangDrums etc) that came with the controler, the akai website says I have to register my product. that's fine. the thing is, it's asking for the actual controler's serial key, wich I guess is something that never came on the box, or that I'm not finding it.
So I need someone to confirm to me where that key comes, within the box, a paper? in the controler itself?
I dont seem to find it, altough I'm 100% sure I have everything that the box came with.
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