You need to go to the workshop "manually", not by using the map but by clicking from screen to screen, to unlock it.
Seriously the game REALLY needs to direct you to do that. You can ONLY do it in the morning when she's in there, any other time you try and do it you just get a "broken door" response, so unless you were lucky enough to try it the first time in the morning you won't ever think to try it again until something changes- which it never will, because the door isn't "broken". I played through the ENTIRE game so far without working that out, because to me it was just another broken door.
Also, it completely throws the game's events out of whack. After finally finding it, apart from triggering one of Lucy's introduction scenes weeks after I'd already pounded her pussy, the next morning I triggered a pair of consecutive events that should have played out WAY before (thanking you for saving her from the kidnappers, then apologising for walking in on you and Maria)