It is a unique 13- or 14-digit number used for jubeat blog widget. (I don't know if it is also used in the other games.) Since juviewer takes the play data from that widget it requires your friend ID.
To get your friend ID, first make your Konami ID and register your e-Amusement card number to e-Amusement pass website. (Note that Konami ID has to be set to Japanese, or the widget won't work. There are some ways to work around this limitation, but I won't explain it here.) Then click "jubeat" icon below.

Select the second one ("Register to jubeat blog widget service"), and you can see the HTML code for the widget along with your friend ID.

Copy the number after "sid=", and paste it above. That's all! :)