External CSS

How do I insert where the external css can be found, and also the class in the external css?

additional_css=”what do i write here”
class=”what do i write here”

My shortcode:

[advanced-iframe securitykey=”123456789″ src=”http://www.google.com” width=”100%” height=”1000″ scrolling=”no” additional_css=”what do i write here” class=”what do i write here”]

What should be in the external css-file if I only want a margin-top=”-400px”

Thanks in advance

http://wordpress.org/extend/plugins/advanced-iframe/