Urs, there’s a difference between a ready-made blog at wordpress.com (such as your new one), and a blog using the software developped by wordpress.org The plugin you mention is for folks who use the software on their own server.
I think it is good that math doesn’t come out perfect here. It should serve to remind you that if you go through the trouble of exposing a nontrivial piece of math discussion, it is a waste to do so just in a blog comment. You should do it on the nLab instead and point people there. ;-)