You will simply need to log into https://fileconnect.symantec.com. Once you provide your serial number, you will be presented with the option to download Symantec Mail Security for Microsoft Exchange 6.5.7.
Here is a link to the Implementation Guide:
ftp://ftp.symantec.com/public/english_us_canada/products/symantec_information_foundation_mail_security/6.5%20_smsmse/6.5.6_build_268/SMSMSE_2007_2010.pdf