If you're using nmap, MAC addresses are only available if you're on the same network segment as the target. Newer versions of nmap will only show the MAC address to you if you're running as root.
i.e.:
sudo nmap -sP -n 192.168.0.0/24
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…