For anyone reading this in the future wondering how to get in, I was able to by opening version 0.8.2 (its what I had from before, no clue if it works for other versions), making a new save (if you have an old one from then it should probably work too honestly) which has to be the same type as what you play (story, infinite, etc.), buying a hook and grabbing the card like intended, putting it in the mailbox and saving, then close that version and open up your 0.9b version, click the "fix pre 0.9.0 mailbox data" button in settings under misc, hit apply, then load into your game, it should hopefully be there ready to go (if its a white circle "not a prop" item then either you forgot the fix mailbox button or it didn't work for some reason, leave before it autosaves so it doesn't get stuck as that error item).
Viewing post in Hooking the keycard in the bunker
Download the old version, i was able to grab the keycard fine in 0.9i, but don't overwrite the current just have 2 votv installed then start a new save on the old version, run and grab some m.r.e inside the log next to the river and sell them then buy the hook and grab the keycard then put it in mailbox. quit old version, start new version and grab keycard from mailbox in your current save.