You may need a Tap instead of a span port. lebrasseur is correct. A span port must be set up properly first, however span ports DO NOT see all traffic because the switch will drop packets that are too short or other types of errors.
You need to read this wonderfully written pdf on the subject of Taps Vs. Span: http://www.netoptics.com/products/pdf/taps-and-span-ports.pdf
Specifically, read the paragraph here from that pdf->
"...In contrast, a monitoring device connected to a SPAN port on a switch does not see all traffic. Corrupt network packets, or packets below minimum size, are usually dropped by the ingress ports on a switch; corrupt packets visible to the monitoring device are usually generated within the egress segment. In addition, switches eliminates layer 1 and select layer 2 errors. Without this information, it is impossible to properly troubleshoot common physical layer problems such as bad frames generated by a faulty NIC.
Additionally, the 5509 is a store-and-forward switch. Here's the doc that says that -> http://www.cisco.com/warp/public/cc/pd/si/casi/ca5000/prodlit/5509_ds.htm
