When you publish, it only asks for the name of the sharepoint site itself, so if you sharepoint site is http://samiam.sandy.com/sites/coolsite/infopathlibrary/forms/allitems.aspx, then you just use http://samiam.sandy.com/sites/coolsite. After it connects to the site, it will ask you if you want to publish to a doc lib or site content type. You choose doc lib, and then it presents you with a list of available doc libs. You will select the existing form library and select the radio button for "update form template." You then finish out the wizard, and it should work.
If you paste in the whole URL to the form library, it will cut off the end for you and just take the site. I don't know where you'd get a message saying "no library exists," because it doesn't ask you to give the link to a library. It only asks to give a link to the site, so is your error saying that the site doesn't exist, or are you getting this message through some other process other than File > Publish?