File unblock property

I have received a ".chm" file from a software developer but i am not able to open it up. I have contacted him and he said that to right click on it, select properties window and click on "Unblock" button. My problem is that i am not getting "Unblock" button in properties window . Is there something wrong in my case?? I am running Windows XP Pro. Help regrading this will be appreciated???

Since it is developed by a developer i don't think that any one out here might be able to help you out unless you mention the name of the software.

Usually there is no unblock button in properties window of any file. They have common attribute like hidden and read only.