I've not used Jxam or Jcertfiy50 -- However I wouldn't worry too much about practicing to use a mock exam that has a similar interface to the real thing.
I'd worry about using a mock exam that has questions of similar difficulty to the real thing. You can find lots of
Mock exams here. Before you start the
test you'll have the opportunity to go through a walk-through of the simulator -- how to check / uncheck your answers -- how to mark them so you can go back to them (even if you don't mark them you can still go back to them during the end review). It's all pretty simple and intuitive, so I wouldn't sweat it too much.
If you want to visualize it -- you've got a question; a few possible answers (either radio buttons or check boxes); the timer in the corner; a check box called "Mark" to mark the question to come back to; And if there's a large code snipped (say more than 10 lines) it comes up in a popup window called an Exhibit. It will tell you how many of the possible answers
you should select
i.e.
Which of the following is a valid
Java Identifier? (choose five)
a- QWERTY
b- $money
c- �ccent
d- 49ers
e- seventy9ers
f- _underscore
g- dot.blah
If you don't choose 5 (choose less or try to choose more) it will give you a warning.