According to the screenshot, it's used as a Winamp DSP plugin. I did a quick test with the latest StereoTool downloaded from their web site, and it appears to be working fine - please see the screenshot attached.
If in your case the plugin works, but you don't see its window - maybe it's somewhere off-screen, try using Alt+Tab to see if it actually presents. You may also want to update your StereoTool installation.