If multiple MAC addresses are found for network devices that are scanned via SNMP, these should be listed at the bottom of the asset summary page in the Network Interfaces section.
If for some reason this information is not being retrieved via SNMP, or you want to display this information higher up on the asset page, you could use Asset Custom Fields for this. Asset custom fields can be customized under Configuration\Asset Pages, in the Asset Custom fields section. These custom fields are only visible on asset pages when filled in, you can fill them by clicking Edit Asset in the left hand pane of asset pages.
