That plugin works fine with Q2A 1.7.0. I tested it with Snow and SnowFlat themes. You just need to enable it in the plugin options: first check the Enable signatures checkbox and then each of the post types you want it to be added. Then, each user adds their signature in their profile page and saves it.
If this doesn't work, then you might have a plugin/theme that is creating some kind of issue there that is specific to your setup.