I think flexible netflow is currently only supported on the 3750-X line.
The configuration is explained here. http://www.cisco.com/en/US/docs/switches/lan/catalyst3750x_3560x/software/release/15.0_1_se/configuration/guide/swmnetflow.html
3750s do not support Netflow, unfortunately.
Does this mean I have no method of monitoring network traffic with NetFlow ?
Not with your own devices, no. There may be some third-party thing out there, but nothing built into the switch.
We leverage our provider's MPLS routers to pull Netflow - it's convenient.
Check with your provider. Find out what device sits in the middle of your MPLS network (if all the MPLS pipes are handed out by the same provider, that is).
If your device is not NetFlow capable you can always leverage our NTA collector and nProbe. Check out this post; Orion NTA and nProbe: Analyzing bandwidth hogs without flow-capable network equipment
As Jeff mentions above you can use a SPAN port. The 3750 allows for the setup of two monitoring sessions and each session can be used to monitor a range of ports or VLANs. You can get more info at the following link which also has a link to a free tool which we developed which can be used to setup SPAN ports.
Once you have your SPAN port enabled you can use nProbe to convert the packet data to flow records or use third party products which can integrate with Orion and do deep packet inspection on the packet data. This approach provides more detailed information which can be useful for troubleshooting. A sample of this can be seen at this link