JVR: There's a "default" field that you set as well, so you can put in your value for when people arrive there organically.
Dave: The parameter name is the variable in the URL string, not the actual value. For example, if your URL is: http://www.mysite.com/?lead_source=mktSocial -- the parameter is "lead_source" and you can swap out mktSocial with whatever you want and it will be pulled into the hidden field. You just have to make sure you stay consistent with "lead_source" or it won't find the value to pull in.