If you have more than 5+ playthrough's, "???" mode should have been unlocked automatically after the 5th one.
Based on reports from other players, the only 2 ways that ??? mode would remain locked is if the player used "save/load scumming" method to quickly get achievements, or if the player didn't truly get to the "end" of the playthrough, i.e. sent back to main menu which is needed to grant certain unlocks and trigger counters.
I'm looking into adding a patch for the next update to help address the issues I described above. However, if neither of these situations apply to you, please do let me know.