The secret code from the last devlog can be inputted anywhere in the game (preferably the menu so it won't mess anything else up). After you type in the code, press enter and you should gain access to a secret. After it has been activated, use ` to open and close it.
With regards to the "Beta Tester" code for the first game, I haven't touched the code in over 2 years so nothing should have changed and it should still work.
Hope this helps!