I'm pretty sure your box is greyed out because you are not able to edit your hosts file on Windows. This is normally a permissions error either because you are not running TU as an administrator or because your hosts file is protected by the operating system (I think Vista was the worst offender for doing this).
HOWEVER, you don't want that option set anyway. It is a bad idea these days to keep your hosts pointing at Cydia. It saves a lot of headaches if you leave that option alone so that you are pointing to Apple. If an when you need to use your blobs in the future, you do so using TinyUmbrella. Just save them for now. Worry about using them in the future when you have to!
HOWEVER, you don't want that option set anyway. It is a bad idea these days to keep your hosts pointing at Cydia. It saves a lot of headaches if you leave that option alone so that you are pointing to Apple. If an when you need to use your blobs in the future, you do so using TinyUmbrella. Just save them for now. Worry about using them in the future when you have to!