First advice: Take a hotel and be there the night before, so you arrive well rested for the day.
On the day itself: First thing you have to do, is go to the doctor for a medical check-up.
After that you go to IBF itself on a different location. The atmosphere is quite relaxed I have to say. When I went, I was there alone. You sit in a room with 4 or 5 computers. The guy from IBF will explain you how to use the computer and then off you go!
There are many different tests, like multitasking, concentration, recognizing shapes etc...
Also personality tests have to be done.
All in all, I did not find the tests that difficult, and there is not really a specific preparation you can do. If you play a computer game from time to time, you should be fine. Only thing is that it takes a long time to complete all the tests, and you need to keep your concentration up at all times, so what I did was only eat little portions in between the tests and the break at noon to avoid that lazy feeling after dinner (I only ate bananas that day;-)
Once all the tests are completed you go for a chat with the psychologist, takes about 30mins, he will ask you how you think you did the tests, wants to know your motivation for joining Cargolux etc..., and that's it for the day.
I got a call from Cargolux 1,5 week later telling me I passed the tests, and I got invited for an interview in the next coming weeks.
If there is a Cargolux pilot who wants to share a few of his rosters with me, please feel free to PM me.
Good luck!