Bootcamp cant use USB stick to download drivers.

Good Afternoon Juliendls,

The resources below will explain how to setup Boot Camp using a USB. What version of Windows are you trying to install? Is it possible that you have not formatted the USB media for Windows?

How to install Windows using Boot Camp - Apple Support
https://support.apple.com/en-us/HT201468

Frequently Asked Questions about installing Windows 7 with Boot Camp - Apple Support
https://support.apple.com/en-lb/HT202008

Where can I get the Windows Support Software (Windows drivers)?

Use Boot Camp Assistant to automatically download the software for your Mac and the versions of OS X and Windows you are using. Refer to System requirements for Microsoft Windows operating systems for more information.

How do I use the Support Software I downloaded from the web page?

1. The download file is a .zip file. Double click it to uncompress it.
2. Copy the Support Software to the root level of a USB flash or hard drive that is formatted with the FAT file system (see question below for steps on how to format).
3. Attach the hard drive to the USB port of your Mac after Windows 7 is installed.
4. Double-click the Boot Camp folder, then locate and double click the "setup.exe" file.
5. Installation of the drivers can take a few minutes. Don't interrupt the installation process. A completion dialog box will appear when everything is installed. Click Finish when the dialog appears.
6. When your system restarts your Windows 7 installation is done.

How do I format USB media to the FAT file system?

Use Disk Utility to format a disk for use with a Windows computer.
Important: Formatting a disk erases all the files on it. Copy any files you want to save to another disk before formatting the disk.

1. Open Disk Utility.
2. Select the disk you want to format for use with Windows computers.
3. Click Erase, and choose one of the following from the Format pop-up menu:
   
   • If the size of the disk is 32 GB or less, choose MS-DOS (FAT).
   • If the size of the disk is over 32 GB, choose ExFAT.
4. Type a name for the disk. The name’s maximum length is 11 characters.
5. Click the Erase button, and then click Erase again.

Hope that helps ...

- Judy

Is your USB a USB2 device or a USB3 device? You should use a USB2 device.