We had a site that advertisers could not target within their AdWords account. Google sent us a response as follows:

I would recommend the advertiser who wants to target your site to take the following steps

1. log in to the AdWords account
2. click on the site targeted campaign
3. click on the site targeted Ad Group
4. click on the blue 'Edit Sites and CPM' link which is located above the
list of sites
5. add the sites the advertiser wishes to target in the box which appears
6. click 'Save'

Thanks for your cooperation.

WHAT WE DISCOVERED, however, is that you can only get to this "Edit Sites and CPM" page after you have set up a campaign. We now tell advertisers to set up a campaign targeting travel.com, which can be selected. Then, when they get to the end, after submitting, from the AdGroup summary page, click "edit" for the "Sites" entry they have just created. THIS takes them to the "Edit Sites and CPM" page google references in their response. On this page, you then delete travel.com and add the site you want to advertise on. Click "submit" and apparently they are then set up to advertise on your site. What is yet unclear to me is if this will result in getting added to the system overall at some point.

Our site is well established. I can't imagine why it is not in the network. Thus, I expect this problem is rather widespread.