This is not Apple, just a community of users so we don't represent the company! I am guessing you may have OS X 10.7.X Lion, mirror requires OS X 10.8 Mountain Lion which is a purchase from the app store not an upgrade. You MacBook Pro certainly is one that supports mirror hence me thinking software.
If not post again and we can look at other possibilities. If not OS issues firstly check MBP and ATV are on same network, apple ID with Home Share on. A reboot of ALL devices including router can help too (take ALL cables out ATV for 30 Secs)