how to use ink refill?

Trying to learn how to use a fountain pen. Got one
 from eBay. The vendor sent 5 refills, but there wasn't
 one in the pen itself.  There's a little plunger thing
 in the pen, in a little cylinder.  

 I'd buy one from someone in Real Life (tm) but haven't
 seen anything remotely affordable.

 Any advice appreciated.