As this is my first plot I'm somewhat confused as to the role that the TCG cards may play. Were these cards part of previous plots? Can someone give an example of how they may be used?
Well, the TCGs haven't been around for all the plots. The ones I remember having TCGs were the ones that had comics. Usually, they hint at the story line and offer a sneak peek at the artwork of the comic - sort of semi-spoilers. For an example of how a TCG deck was related to a previous plot,
Curse of Maraqua comes to mind. The
TCG is here. As a specific example, you could tell that there would be a new Maraqua by the flavor text on Isca's TCG, "Isca was among the thousands travelling to New Maraqua to seek a new life." They choose phrases for the cards that that will create interest in the plot, and are particularly good at choosing those that will trigger some creative speculation, but are very careful not to give too much away.
In this plot, several of the images from the TCG cards have been in the comic. I don't remember that happening before, but I wasn't paying as much attention to the other plots as they weren't "mystery" plots. We have also seen the cards' subjects and images in places other than the comic (ie. Temple of 1000 Tombs and its traps.)
So, as Sky answered earlier, we use this information to speculate about what will happen next, both in the comic and with the other aspects of the plot. We can also use it to gain a little better understanding of what has happened. As an example of both, see
my post on page 3 of this split.
So, how do you know when a new TCG card comes out? Do you just keep checking the checklist?