I installed it and it like starts to launch but never actually does? I uninstalled and reinstalled it and it still isn't working?

Just bought the game. It launches in a small window and ALT + ENTER doesn't work...

I missed a single firefly in a level and sadly there's no warning when you enter the next level. Now is there no way to go back to recollect that missing firefly, or do I really have to restart the whole game to get that missing firefly?

Is this version only playable online? It never works offline for me.

According to the manual, to enter a code you select "Continue" from the main menu, but every time I do that it just loads my last location, wherever that was. I'm trying to repeat a level to get 100%, and have the code for the start of it, but can't find where to type it....