This should be fixed in the latest release. Make sure you have upgraded to version 3.0.2. Also, consider using the keyboard shortcuts for kerning instead of the kerning-field; they make kerning faster and easier:
Did you relaunch Glyphs? Glyphs needs to be quit and relaunched after a plugin was installed for the plugin to be loaded. If that does not help, try installing it again from the plugin manager. It works for me.