I don't know why there are no urls posted, but it may not be allowed. Go to the read me (in ssu file)-> Follow the link for For new installation->on that website in requirement 3 is a link to the module manager.
You have to install module manager to get the add on to work. The yclass.php file is in module_manager_rev16zip in includes-classes.
There is a newer version, but that file is missing. I don't know if that was a mistake or if you may not have the newest version. The newest version is Simple SEO URL3.7.1, but the one you can get from zen cart is Version: 3.5.8.
If you have the old version the link is bad, but choose ssu and installation on the left to get the newest version and the link to module manager.
I would try 3.7.1 with module manager version 17. If that does not work remove version 17 and use version 16.
I am using 3.7.1 with revision 16 of the module manager, they work as long as you are not using php 5.3. They are causing me trouble on php 5.3, but I have not been able to resolve the issue. That might be on my end, but I am not sure. Good luck.
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