Note: Check the comments section - there seems to be a very live Q&A discussion on the topic.
Boot Camp Assistant requires an Optical Drive with Windows installation in it so it can install Windows.
For Macbook Air and others it supports an ISO option and it creates an usb installer.
This is how you can force Boot Camp Assistant to support the latter option for Macbooks that are supposed to have optical drives but you don't or can't use it:
Get the following strings from System Information:
Get the selected one and the second line also (MacBookPro8,2).
Save a backup:
sudo cp /Applications/Utilities/Boot\ Camp\ Assistant.app/Contents/Info.plist /Applications/Utilities/Boot\ Camp\ Assistant.app/Contents/Info.plist.bak
Edit with root permissions (either change the permissions of the file or sudo vi) the following file:
sudo nano /Applications/Utilities/Boot\ Camp\ Assistant.app/Contents/Info.plist
add your string (mine is MBP81.0047.B27):
DARequiredROMVersions MBP81.0047.B27 IM41.0055.B08 IM42.0071.B03 IM51.0090.B03 IM52.0090.B03 IM61.0093.B01 MP11.005C.B04 MB11.0061.B03 MBP11.0055.B08 MBP12.0061.B03 MM11.0055.B08
Then find the following:
PreUSBBootSupportedModels MacBook7,1 MacBookAir3,2 MacBookPro8,3 MacPro5,1 Macmini4,1 iMac12,2
REMOVE the "Pre" from the and add your computer as a string, mine is MacBookPro8,2:
USBBootSupportedModels MacBookPro8,2 MacBook7,1 MacBookAir3,2 MacBookPro8,3 MacPro5,1 Macmini4,1 iMac12,2
Save and exit with <Ctrl+X>, Y, <Enter>