Do Daum and Nate have their own ad platforms? Can you briefly describe how they are similar/ different from Naver?

Yes, they do have their own SEM platform.

That is called "Daum Clicks" which has launched since 1st June 2011. Through this platform, you can do SEM in Daum along with Nate and Yahoo altogether at the same time.

This platform is running in CPC pricing model and as you can see from the image below. It will be the second section in the search result page.

Naver is solely running the SEM program by themselves from 2011 Jan without Overutre's sponsor link section. So if you use Naver's platform, your ads will be shown right under the search box.