How can I enable netflow on Cisco 3550 or Cisco 3750 devices?

No. Cisco doesn't support NetFlow on the following devices:

• Cisco 2900
• Cisco 3500
• Cisco 3750

 It is possible to configure NetFlow export, but no flows are send to the collector. Only the first packet of the data flow was exported in the older IOS version.

As alternative you can use FlowMon probes. The probe sniff all traffic going through the line. The main advantage of probe is its ability to analyze every packet at wire-speed up to 10 Gbps, it is L2/L3 invisible and can be connected to any point of your network. The probe can be connected to the user network in three ways: via mirrored port (SPAN); via Ethernet splitter (TAP); or via built-in splitter. See the following URL http://www.caligare.com/product/flowmon/ for more information about probes.