The first thing to do is reach over and punch the person who wrote a plugin without writing a readme
Then step 2 is to search on the forum for the thread about the plugin. DonationCoder is such a "chatty" community that we sometimes forget i think that anyone uses any of this software who isn't daily visiting the forum and participating in discussions. So the Printerlist Plugin forum thread is: here
I think that thread tells you how to basically use it (and install it).
Alternatively, you can if you cant find a thread on a plugin, go to the plugin options tab, and open up the plugins configuration form, select the plugin and look at what it has listed as the trigger regular expression or keyword.
Lastly, apologize for punching the plugin writer and give them a hearty thank you for taking the time to write something nice and share it with all of us.