I'm sorry but I don't understand which problem are you left with?
The not authorizing problem (because I've had that and solved it) or are you just wondering why it's downloading the entire game again?

I'm not left with any problem now. It wouldn't let me import the folder for some reason, but when I installed it checked local files and then installed everything again - which has been explained sufficiently by MrBMT.

I don't know why I'm not authorised to import the folder for this game, but it doesn't matter any more.

Thanks.