The interface for this is a lot less friendly than the introduction screen. They also ditch the overall bright theme and rounded, friendly font for a more regular grey-ish background.
There seem to be a lot of options here. I never realized that there were so many different types of ways to pay for the subway. But like most Boston commuters, I already have a CharlieCard and I use that to get around on the subways and buses. Today, I just plan on refilling that card. I tap the button for the CharlieCard several times as it doesn't seem like the touch screen is that sensitive.
Whoops! Turns out, you're just supposed to tap your CharlieCard on the card reader on the right. I should've read the instructions more clearly.