YES, you can put it below. Invisible may be a problem with google and some other search engines if you are hiding or spamming key words. They could ban you.
I would just look for a place high up in your copy to place the major keyword, but don't go to crazy, your title tags are very important as well as inbound links using your keyword in the anchor text. This is all for
SEO.
You can create sub pages regarding a particular keyword and put that on your website and optimize it.
Here's a great article on inbound links and anchor text
Inbound Links and Anchor Text
and an xml tool to create a sitemap so google can crawl your pages faster
Create your Google Sitemap Online - XML Sitemaps
Hope it all helps
Frank
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