I am having an issue working with Single Sign In. I have the plugin installed and configured with the default settings on the Master and 2 Slave sites (This will eventually need to work for about 100+ sites). After I have logged into Slave 1, I visit Slave 2 but the user (A Super Admin) is not logged in. I checked Slave 1 for the SSI script (pointed out in the SSI Documentation under "Troubleshooting"), it doesn't show up anywhere in the page source.

I suggest that you review the trouble shooting again.
If the call to the other site is not present in the page then try to setup manually the domain for which you want to have the "ping".

Also ensure that the plugin is enabled in each sites where you want a ping is send