Does your MAC have multiple ethernet ports? I ask because if you connect the two directly together then how will your Mac be on the Net?
You didn't say if you have 100 or gig ethernet but you could run down to your local Walmart or similar store and pickup a cheap 5 port switch and install it near your Mac and connect the BB there for ethernet access.
If you only have 100 speed then you can pick up a 5 port switch for $15 or less. Log into Walmart.com and search for "Ethernet Switch" and you can check to see which items your local store has in stock. Can also try Radio Shack, etc.
Using a dedicated switch will be a far better way to go and won't require the MAC to be left on 100% of the time.
Devices Boxee Box: March 5th release, Ethernet / WD Live (Gen 3) via 802.11n
Internet: Verizon FIOS and Comcast Internet (both active)
LAN: Wired Gigabit and Netgear, Verizon, Belkin Wireless Routers
Storage: WD NAS(s) & Windows Server(s) (2012) 10TB