I figured out why the usb flash wasn't on my start screen, I had it connected to power supplied HUB, when i connect the usb flash directly to my imac usb port i got it on my start screen.
Still, as soon as the "choose a drive to install windows" menu pop up, i got another problem: the "install" button on bootcamp was off, I had to format it to nfts and then yes i was able to install windows.
I was able to install win 10 without install win 8 first.
Maybe it didn't work from the beginning because i was running mavericks instead of yosemite (that kinda *****, because i don't like yosemite. I also hate all windows after XP, so it's also sad i cannot have it on this machine, it works on my old 24' imac though, but the screen is all yellow)
Maybe I should unplug all the external hard drives and have my usb flash on directly connected to my imac usb port.
Thank's Loner T for your help, I hope this help other people struggling with the same problem, sorry for the bad english, and if someone is going to install windows 10 as far as i can tell it's a free UPGRADE, wich means you gonna need a previous windows installed with a valid serial-number or you will have to buy a serial-number for your win10.