I dont normally use those logo's, but reading your post I thought i would try it.
No idea if your having trouble still but i did the following.
I looked at the link from Basara, you will need that.
Then I found this resource from comodo
cant post linkshttps://www.positivessl.com/secure-site-seal.phpcant post links
You choose your cert type from the list, then choose the code size, it tells you to save the pic by right clicking etc, i didnt bother. But i am sure there is a reason to do it!!
Anyway
Copy the code from the page i linked to, then make a HTML block as shown in video link, make sure your not in visual mode and simply paste the code from comodo site into it.
Then all I did was pick the home page from the layout menu, and in a random location chose to put the HTML block I had made. I then clicked on the little enable button on the block and save the layout.
The screen shot shows it working, keep in mind it was a quick try and the placement was random!!
I will redo it properly at some point if i want the seal on my site, otherwise i am taking it off, but as it shows, it is really simple to do.