You can't do it in the first attempt?
-It's Normal and even reassuring ; what must be done is not natural, So you have a brain that works well and that we can't fool easily. Congratulations.
However, if your brain relaxes for a moment in order to let your eyes transgress the rules and see in 3D, he'll realize that it does not hurt and even that it's pretty cool. It will be then very easy to do it again.
Here are some tips to facilitate this first viewing:
1 - You must touch the screen with the nose
2 - Each eye must be in front of a circle
3 - Think to almost close your eyelids before and while you move back of the screen
4 - Take your time, especially while you move back
5 - Relax! it is crucial (If you can't or don't want, you are definitely in the wrong site. We can direct you to this one)
6 - It's easier when you're alone, to relax and let go without judgment
7 - It helps to understand the method, with the animation and the detailed explanation (linked here)