The product has no integrated update function.
Checking today ftp://ftp.nai.com/pub/antivirus/datfiles/4.x/ shows a valid avvdat-7312.tar file present, so the site looks good.
Suggest you take a look through McAfee support article:
KB67513 - How to download the AVV-DAT package for use with VirusScan Command Line Scanner 6.x for Windows and UNIX
This article also includes an example ftp shell script you can try (note it's just an example, and is unsupported as a result).
I can't vouch for generic wget functionality, but using --no-cache sounds reasonable given the dat file is updated every day.
Perhaps you have a firewall or external service provider inbetween cacheing data.