The MIT License is recommended for most projects. It is simple and easy to understand and it places almost no restrictions on what you can do with a SyntaxHighlighter project.
If the GPL suits your project better you are also free to use a SyntaxHighlighter project under that license.
You don’t have to do anything special to choose one license or the other and you don’t have to notify anyone which license you are using. You are free to use a SyntaxHighlighter project in commercial projects as long as the copyright header is left intact.