Dreamweaver \ HTML problem

The code to do it manually is actually quite simple.  First, set up your target element with an ID, like this:

 <div id="mytarget"></div>

Note that it doesn't have to be a DIV element if you don't need that, a SPAN or a P will work just the same.  The ID can be anything you want, just no spaces and it needs to be unique on your page.

Now your onclick event needs to be something like this:

 onclick="document.getElementById('mytarget').innerHTML = 'Text inserted using on click'; return false;"

The 'mytarget' has to correspond with the ID you used on your element above.  Common gotchas are getting the quotes wrong, which may not be clear above.  In the above example, because double quotes (") are used to wrap the onclick attribute, single quotes (') need to be used as string delimiters inside the onclick.  The 'return false' at the end stops the '#' link being followed, which you may or may not want.

Finally, I should point out this sort of thing is acceptable for one off use if you're quickly knocking something together or are only doing a small amount of manipulation, but the correct (W3C) method for doing this sort of thing is to create and append DOM elements, but this is a little more complex in the simple case.  Have a read of this article for explanations and alternatives to innerHTML, you might also take a look at a javascript library like jQuery which makes that sort of manipulation easier.