I did a quick search and it appears that 12.2S or later is no longer a supported IOS version.
Cisco IOS Software Release 12.2S, which includes Release 12.2(14)S, Release 12.2(18)S, Release 12.2(20)S, Release 12.2(22)S, Release 12.2(25)S, and Release 12.2(30)S, has reached end of sale and end of software maintenance. For more information, please refer to the following document:
From the Engineering Toolset Administration Guide:
Did you check to see if the OID below worked..
NetFlow Configurator uses SNMP to change the running config of the NetFlow-capable device. If your device cannot be updated using SNMP, it does not support the OID 22.214.171.124.126.96.36.199.3188.8.131.52.1, then NetFlow Configurator cannot update the NetFlow configuration of the device.
The following Cisco IOS versions support the configuration of NetFlow using SNMP:
o 12.4(10) or later
o 12.4(10)T or later
o 12.2S or later
According to what I have found on Cisco's Website it appears that 12.2 SX is a different version of IOS with details below..
I would use either of these reference guides to configure your 6500 for Netflow.