WordPress.com login for Comments

Hey guys, I’m trying to figure out how to make the little button for a wordpress.com login to post comments with? I’ve seen it done before with plenty of sites but I haven’t figured out how to do it to my own site: https://mitomeru.net/?p=17. I’ve tried looking for a plugin but I haven’t found one yet.. maybe I skipped over something.

Also, I am working off of an old theme that was designed for 2.x rather than 3. I’ve had to add in lots of stuff to the theme that is already on themes nowadays by default so maybe it’s just something I’m missing by default.

Any support is appreciated, thank you so much !