Has your 500 been updated to handle ePub? If not, it will never work with a MAC nor will it ever work with a 64-bit Windows as it will need a32-bit Windows only driver to actually talk to it.
If you have not upgraded your 500 to handle ePub, contact Sony and find out about doing so. It's the only way your 500 will still be usable. After the upgrade to ePub, the 500 will show up as a removable drive..