Using Groovy plug parameter makes no Agents available

Hello,
I installed the Groovy plug in following directions (I thought anyway).  It shows the paramaters, including the ones that I added.  However, if I use those in my project config it shows that now Build Agents are available.  There is really no other information as to why--just a "-".

Is there something that I need to put on the Build Agents as well to make this work?

thanks.
--Jim