If you aren't a developer, then you will not understand how to add the license code required for the plugin. The reason they state that it is for developers only, is because it takes understanding PHP code to implement the Software License manager into a plugin to license it.
My suggestion would be to go back to the plugin developer that you acquired the plugin from and have them look at the Software License Manager plugin and add the necessary code to license it. Even if I showed you how to do a general license, you may not be able to add it properly to the plugin you are referring to and you could break the plugin so it doesn't work.
It is always best to have this type of thing done by a professional coder who knows what they are doing. The developer who created the plugin is your best bet for getting this done right!