The pigtail is a little spiral thingy on the bobbin, about half way in the open area. It looks like you thread the bobbin regularly then put the thread through the pigtail thingy.
I have a Tin Lizzie and have been having tension problems. I was wondering if this would help. Sounds like lisalovesquilting gets a better stitch using it. I haven't used it but think I will give it a try.