Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
The 'security key' or 'safety key' is not really a key. In some cases, it is a small disc magnet, in a plastic case, on a string or neck-chain. You place it on the console in the right place, it stays there while you are exercising and allows the belt to move. If you were to fall or faint, the key - with a string or chain around your neck - would pull free and that would stop the belt instantly. That's what it is for, that is why you need to put it on the console.
If you need specific help with this, I comprehend entirely... if the key wasn't shipped with the Trainer 430, or if you can't tell where to put it or insert it, let me know.
(If this answers your question in full, please rate my answer, so that this case may be closed. Thanks.)