It doesn't much matter which you use (kj's or cals) so long as you're consistently adding up the same type (don't go mixing cals and kj's - it's mixing apples and oranges)
I don't think you've been counting in kj's, or if you are reading kj's you've not been eating the portions you think you are. If you were having 2500kj's per day, that's around 600cals which is nowhere near what you need to be eating and you would definitely be feeling unwell. It's also nowhere near what it sounds like it should be given what it sounds like you're eating (in fact one of those muesli bars is about 130cal (540kj) and the noodles are about 300-400calories, 1 "piece" (not sure how big that is) of fruitcake might be about 140cals?). If you're eating 3 meals per day plus 2 snacks, there's no way you would be as low as 600cals in one day unless all you're eating is lettuce and celery, literally.
I don't count cals or kj's because I simply can't be bothered, but I do check them for nutrition and energy content to make smart purchases. To do that you need to know how to read one. When you read a nutrition label you need to look at the serving size, the kj's or cals per serve and I usually look at the "per 100g" for comparisons with other products whose serving sizes might be different.
You can do this one of 2 ways. Only eat in the serving sizes given (these will be much smaller than you think I'm betting, you've probably been adding up the cals for "one serve" but have actually been eating the equivalent of 2 or 3 serves) then tally up the serving size cal/kj values.
OR measure the amount you're eating and then work out how much cals/kj's are in that based on the 100g kj/cal value.
There are also calorie counting diary programs out there that will help you figure out how many cals are in what you've actually eaten, I just can't be bothered with them to be honest. Lots of people swear by them though.
That's the most basic way to use nutrition panels. Once you get that down pat you can start doing complex things like comparing percentages of different types of fat, and sugar etc
The other thing to bear in mind are liquid calories - things like soft drinks, coffee/tea, juice, sports drinks, milk drinks etc etc...
35kg lost. (November 2005 - October 2006)
15kg gained again (as at October 2010).
Back to the drawing board - Let's do this thing!
"You can't change the winds, but you can change the sails"
"Reach out and take control of what lands in your lap"