adding meta tag

Publish your website (FTP, MobileMe or Folder)
Open your Sitename folder ( you can use Cyberduck if you publish to FTP).
Inside, you will find your HTML files.
Open these files with a text editor made for code (Fraise, Textmate, textwrangler, etc.)
In the head part (between < head > and < /head >) paste

<META NAME="description" CONTENT="description of your page">

<META NAME="keywords" CONTENT="write, your, keywords">

Know that [Google doesn't use keywords meta tags|http://googlewebmastercentral.blogspot.com/2009/09/google-does-not-use-key words-meta-tag.html]

You can add those with the free iWeb SEO application. Publish your site to a folder on your hard drive or to MobileMe and then run SEO to add the keywords and page descriptions. If you publish to a 3rd party sever you'd use the publish to a folder method and then upload the site folder and index.html file to your server with a 3rd party ftp client like the free Cyberduck.

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Although SEO has a built in FTP client many have not had good success with it and prefer to use Cyberduck or other 3rd party ftp client.

OT

Publish your website from iWeb to a folder on your Mac.
Open this folder with iWeb SEO Tool and add the title and meta tags and the meta description to the html file for each page of your site.
Now you can upload the HTML files using an FTP application like Cyberduck.

I suggest you use the free editor TextWrangler to do any html editing. But for metatags you should use iWeb SEO as it remembers what you added and can add them again with one click if you do any editing and republishing.