The 5300 series switches don't have explicit CLI commands to configure sFlow. The switches support configuration of sFlow using SNMP and sFlow analysis software that supports the sFlow MIB should be able to automatically discover and configure sFlow monitoring in the ProCurve switches:
http://blog.sflow.com/2009/06/sflow-mib.html
There is also a command line sflowenable script that you can run on Linux systems that will configure sFlow using SNMP:
http://www.inmon.com/technology/sflowVersion5.php
Finally, you can use setmib commands on the ProCurve CLI, but I wouldn't recomment that approach - it's complex and error prone.