Some basic questions need answering first please:
Agent alone is not enough, which MAC VirusScan product is installed ?
Have you added the extensions for the same product to the ePO server ?
Have you defined an update task in ePO ?
If (yes) to the above, what messages are you getting in the agent log at the time of dat update attempt ?
Apologies if these sound very obvious or basic things but we need as much detail as possible to be able to assist.