Q:

How can I cause Snippets to appear in the HTML/WYSIWYG editor toolbar?

A:

The Source property of each HTML field can be used to specify an HTML editor profile which defines what features are visible to which users (see http://sdn.sitecore.net/FAQ/Rich%20Text%20Editor/Editor%20profiles.html). If no profile is specified, the default (/Sitecore/System/Settings/HTML Editor Profiles/Default) will be used. This default profile may not include the Snippets item; to add it create a new Html Editor Button named Codes under /Sitecore/System/Settings/HTML Editor Profiles/Default/Toolbar 4 with Click set to Codes.

To add a snippet create a new Html Editor Snippet under /Sitecore/System/Settings/HTML Editor Profiles/Default/Snippets; set Header to the label the user should see in the toolbar, Value to the HTML which should be inserted into the HTML field when a user selects that entry.