I suspect that attending a diversity fair as a white male may potentially hurt you more than it will help you. Firstly, firms might think you're trying to scam them. Secondly, firms go to these events specifically to recruit minorities. Therefore, they're not trying to recruit you. They might even be annoyed that you "took" a spot that could have been used to interview another minority. You'd have to really impress them in order for them to give you a call-back when they already have hundreds of white males that they're interviewing and they're specifically looking for minorities.
Unless you have outstanding credentials and/or interviewing skills, I probably wouldn't do it. Where are you trying to interview? There are lots of career fairs that don't specifically target minorities.