You can now find a key in the downloads section; use it to activate on lexaloffle here: https://www.lexaloffle.com/redeem
Right -- it seems that itch's external key system is not available for bundle customers.
If you mail me, I can manually activated it though; see: https://www.lexaloffle.com/bbs/?pid=157210#p
Did you get PICO-8 via a bundle? See https://www.lexaloffle.com/bbs/?pid=157210#p