The below is an excerpt from the Q&A on the 3550. The 3550 only provides Cisco proprietary POE. You will have to upgrade to a 3560 or use another power source.
Q. Does the Cisco Catalyst 3550-24 PWR Switch support the 802.3af inline power standard?
A. No, this switch supports Cisco Pre-Standard Power over Ethernet. The Catalyst 3750 Series and Catalyst 3560 Series support the Cisco Pre-Standard Power over Ethernet and IEEE 802.3af Power over Ethernet.