I have purchased a Linksys WRT300N router, and the UPS guy dropped it off but then it grew legs
I was wondering if there is some way that i can identify the MAC address of the router if the thief is using it?
(quietly entering this thread, "Hi, Im new here

)
About your question, You should be able to view the MAC addy if you already have a wireless PC/Laptop and assume someone is using it within your area.
If you already know the (WiFi) MAC address before it grew legs you could:
Monitor available networks , depending on how robust your WiFi adapter is equipped. There should be an option to individually select a specific network and get more info as in - Type of Encryption, Speed, Channel,
MAC address 
Its been years since I did any "wardriving" for my IT classes in college

, - so I loaded my WiFi connection and retrieved more details on each connections available. It showed the MAC address of that router, not the host's MAC address connected to the router. Next I called my neighbor who has a WiFi (encrypted) router. I asked to give me his Linksys MAC address (I informed him on what I was trying to find out). Sure enough, his MAC addy was showing.
The person who took your router could have changed the MAC addy on the router, then Im sorry to say - your out of luck.