Make sure once the plugin is installed, you enable it and follow the directions it gives you:
"WordPress system plugin. To make use of this plugin you will need to enter the URL segment to your blog into the Blog URL path parameter. For example, if the URL to your blog is
http://example.com/blogs/, you will want to enter blogs."
If you don't do that, it will operate as though it is uninstalled.
To download the plugin:
1. Go here and log in if necessary
https://www.corephp.com/members/clientarea.php?action=products2. Click "Download" to the right of "Wordpress for Joomla!"
3. Click plugins, then select the system plugin.
4. Make sure the Wordpress system plugin runs before the sh404SEF plugin in the Plugin Manager
It STILL doesn't work?
Caveat #1: The Blog URL path for the plugin MUST match the menu item alias or it will operate as though your system plugin is not installed at all. You can find your menu item in your Joomla Menu manager. It will be the item with "Wordpress" as its type. This can be modified in the Joomla Menu Manager.
Caveat #2 (not encountered very often): After configuring SH404SEF, sometimes the URL manager will cache "blog" (or whatever your blog URL path is) when it should be "blog/" so it will fail. The trailing slash can make a big difference. This can be changed in the URL manager in SH404SEF. Note: The URL parameter in the system plugin should not have a slash in it though.
Kindest regards,
Andy