The Plugin you are referring is for Premium Wordpress accounts.
You can't use Plugins in Wordpress.com free Blogs. It is discussed many times in Wordpress forums and answer is always that "iFrames are not supported in Wordpress.com free Blogs".
Yes, you can use that Plugins with Wordpress installed websites or on Premium blog account.
I hope you understand me. You can also try the same. It is very simple.
Go to Wordpress.com -> create one account. Then create a Blog and try to add one iFrame inside one Blog post.
It will then automatically remove that after you save the post.