How can I find my key? I went through the e-mails already without a result. I would need the player key to get the validation key to use planets to open skins

Best bet is to contact Ubisoft Support Services at http://support.ubi.com. If you contact by email, make sure these criterias are fulfilled:
o Full name used to order the game
o Order number
o Email address used to order the game

I heard it was possible to extract key information from United if you had Steam, but i guess that's irrelevant.

Please correct me if I'm wrong

And then, how do I know my order number? Seems like I don't have the email left (even though I was sure for that I do have it).

Maybe I need to live without the skins that aren't free

i think they can resend the email if you have the email adress