tuxedo, hope the following helps.
There are two types of ad placements, site wide ones which Google automatically creates for each domain and optional custom channels created by site owners.
As we have discussed I suspect AdWords advertisers can only find either type of placement in the following tab:
Display Network > Change Display Targeting > Search for placement ideas > search box
if each has sufficient traffic as those are the only ones of mine that show up for me.
As RhinoFish points out, if your site shows up to an advertiser in the automatic placement report, the advertiser can add it as a managed placement. It appears this is the case even if it cannot be found in the above described search for placements tool.
Also advertisers appear to be able to target the site wide and perhaps custom channel placements even though neither is returned in the above type search by going below the above mentioned search box to the "Add multiple placements at once" feature. There one can manually enter domains and site pages.
Keep in mind just because you can add them, does not mean it solves the overriding problem. Meaning, I was able to add domains of mine as managed placements in AdWords that don't show up in the managed placement search using the "Add multiple placements at once" described above but when I hover over their status icon they state my ad is not showing due to "This selection has very low ad traffic".
Also if the advertiser has any keywords in their adgroup and Google does not think they are relevant to your site, the advertisers ad may not show on your site even though the advertiser has managed to add it as a managed placement.
To see if anyone has ever targeted a custom channel, login to AdSense and click the Performance Reports > Targeting Types and select All Time for the date range and if anyone has targeted one of your custom channels "Placement" will show up in one the the report rows. If it shows up, click Placement and then change it from Days to Custom Channels and you will see a breakdown of the custom channel placements. Unfortunately site wide placements that have been targeted do not appear to show up in this report.
Still wish either kind of placement could be found in the search box regardless of traffic as that is the most intuitive way people are going to look for placements but for advertisers that inquire though your site you could tell them how to add your site manually and just hope you have or eventually get enough traffic for their ads to show.