I can't install or remove windows 7 or later using Boot Camp Assistance?

I tried for a couple days to install Windows 7 on my MacBook Pro which has OSX Yosemite (10.10.2). As you see in the capture, the third checkbox is disabled. I create a Windows 7 on a USB flash and downloaded the latest Windows support software from Apple, all I have to do is install it on the Mac.

What's the problem here?