There should be a link on the page that allows you to contact the owner of the website to request membership to that tree...I know that has worked for me in the past when I have several matches in someone elses tree. The worst thing that can happen is that they deny you access...defeats both of your purposes in my mind.
Patrick is correct. Look for the Contact Webmaster link located just above the list of your matches.
The reason why you cannot access certain websites is because they have been set to Private and only allow invited members to access the site.
The best way to approach these people is to write them a personal note and let them know you found a match in their tree. Keep in mind that people are sensitive to privacy and it helps a lot if you have some information to offer them or an interesting story to tell.