We will be happy to help!
Did you go through the Salesforce documentation
There are steps that you will have to perform from the Salesforce administration to run the plugin.
The most significant one is to install a special package to add required custom fields to your Salesforce objects. Then you will need to obtain a security token, which you enter in the plugin settings from nopCommerce administration.
You can look the Installation and setup section from the Salesforce documentation for more detailed information about the steps you need to perform.
Hope this helps!