For your keywords and descriptions, you can do that in "edit" / "edit keywords and descriptions".
Is there a limit?....in a sense, yes. Keywords have been ubused, so use then only as they fit the content of your page. Repeating the word "horses" 525235 times just to get a better search listing for things about "horses" won't work. Use the same words found on your web page, and even if using strings, try not to repeat any word more than 3 times.
Keywords can be separated by commas. For strings, just omit the commas...no quotes needed that I know of.
If you want to say more than the single words, use the "description" which can be a 'short' paragraph, or better to find one sentence that describes what your page is about. Abuse of 'meta tags' will hurt your SEO more than lack of using them. They are no longer as important to search engines as they used to be because of the overuse or abuse of them.
As for the "Header" Information, well, that can be a lot of things.
Tipical Header info can include style sheet information, tags relating to encoding (even tho they are 'meta' tags) and even the "title" that you see at the top of a browser's window or 'tab' that says something about the web page. You can also have information on using javascripts and links to outside style sheets
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<title>Quiz Answer Sheet WHMIS</title>
<script type="text/javascript" src="swfobject.js"></script>
<link rel="stylesheet" href="index_style.css" type="text/css">
That is an example of some stuff that could be inside of a "Header information" area.
Hope this helps. I can be a bit confusing at first. Start with the keywords, and a short description. Those are the things that can help somewhat with search engines. The Header information is only needed if you are using it, such as the link to an outside style sheet, but VSD does not produce one anyway.
E-Learning Specialist
www.mainsites.ca is my website, and yes, some of it is crappy.