Do you have fairly current firmware in that switch? The latest is 2.72, released Sept. 2004. Monitoring may not be available in older firmwares (mine has 2.71).
In the web GUI, click Configuration on the left-hand side, then click "Roving Analysis Port." Then change "Roving Analysis State" to "Enabled," and select a port to monitor (looks like you can only monitor one at a time) and a port to mirror it to (analysis port). If you have a stack you should be able to have a monitor port and analysis port in different units. Pretty cool.
good luck, Jonathan