Unfortunately, I have some bad news for you. The EWS 5.1 software is End of Life, and has been for over a year now. Even more unfortunately, EWS 5.5 has been end of life for a while as well, so getting the iso for that version is going to be difficult at best. EWS 5.6 is still supported, but I strongly recommend that, if you are just using your appliance for email scanning, you upgrade to at least McAfee Email Gateway (MEG) 7.0, if not MEG 7.5.
Either way, since you are having trouble getting the necessary download, I recommend getting in touch with Support to get assistance with finding the download you need.