Or maybe, like myself, they do not realize that the “Like it” option has to do with the monthly contest.
A word of clarification. We do not use the number of "likes" and comments as an absolute judging criteria, even though the sum of these interactions is taken into account by the jury as a "popular vote". Our contest rules say:
"User commentary and ranking gathered by the competing entries will be considered by the jury."
...thought that adding a comment was as good as a vote...
It certainly is. The advantage of comments over likes is that they obviously provide more input to everyone concerned than "liking". I often think myself of a "like" button as a mini-comment in a rush. Of course you can feel free to use both to double the effect :). I guess that using your personal style is the way to go.
The like button is a fairly new function at iModeler. An aspect of its use may be the placement of the button, we have considered moving it together with the comments stream. Everyone's further thoughts on this would be appreciated.