If it is anything like the other PSUs I have opened, you remove the top case of the PSU (after you have unplugged it from the wall of course!) and the fuse should be pretty apparent. It should have markings on the ends that indicate the rating. Just be careful though, unless you had a power surge that blew it, the fuse going is likely to be just an indicator that something further down the line drawing too much current. If that is the case, the replaced fuse will most likely blow too.