That option is only available for Macs without a built-in SuperDrive (the MacBook Air, Mid 2011 and newer Mac minis, the Late 2012 iMac, and the MacBook Pro with Retina display).
If you haven't got one of those computers and Boot Camp Assistant doesn't show the option to install Windows 7 from a ISO file, your Mac doesn't support the Windows installation from a USB because it's restricted by firmware to a Mac without a SuperDrive. In this case, you have to buy Windows 7 on a DVD from eBay or Amazon, and use it to install Windows through Boot Camp.
If you still wants to install Windows from a ISO file, the only option is to install it on a virtual machine, using an application like Parallels, VMware Fusion or VirtualBox