Windows Assistant doesnt find Drivers on my Bootcamp USB Stick

I have iMac (21 inch without optical Drive) with Mavericks. I wanted to install Windows 7 home premium 64 with a 8GB USB Stick. The Partitation workes fine with Bootcamp and it creates without Problems an installation Stick. After i choose the size for the Windows Partition, i make 100 GB. Then as written in the Manual, the iMac reboots at the End and starts with Windows 7.

So Far so good.

The Windows Installations Assistent wants me to choos my Language and then it opens a Window with the Error Message: He cant find Drivers. A new Window opens to search for them. But i cant select them because it doesnt show the Bootcamp Stick only the empty Windows Partition and the OSX Drive. After looking everywhere for them i stopt the installation and rebootet with OSX and tried everything again. But it endet the same Way...

Whats the Problem, why cant it find the Drivers on my USB Stick? Do i need Yosemitee or an other Bootcamp Version?

Please Help me! Thanx!