selenium IDE 101 - how to visit any page
The answer was that you put the Base URL up top, e.g. http://www.google.com and then you put the rest of the url, e.g. /images in the 'Target' field.

If you only want to go to the base page of the site (e.g. "http://www.google.com") you just use that for the BaseURL and then use in the 'Command' line use 'open' with no parameters) but keep the Target field blank.