Inserting HTML code

We’ve had our WordPress site up & running for a month or so, now. We need to insert some “validation” coding, to enable search engines to work with our WordPress site. We have some coding we used on our website, before we switched to a WordPress format. This has worked well for us, and we’d like to use it again. I have tried going to the HTML tabbed window for a page, and inserting the copied text. However – when I try to view the change (in the WP page editor window), all I see is a small piece of the HTML coding I’ve just tried to insert. It seems that virtually all of the code I’ve just tried inserting, has ‘vanished’.

Can anyone please tell me, how we might insert some coding, and not loose it? What am I missing here? (All help gratefully received!)