Click on File > Import menu item in postman as displayed in the image below.
Once clicked on Import menu item, a pop-up window will be shown as displayed in the image below to import postman collection file.
In the window similar to one which is displayed in the above image, click on the Upload Files button under the File tab. Select the PublicWebAPI_Postman_Collection_vx.x.json file from the extracted download archive file which you downloaded from NopAdvance website to install the plugin and click on Open button in the file explorer window.
Once the postman collection file is selected and opened, a pop-up window similar to image displayed below will be displayed.
Click on Import button displayed in the image above. This will import the postman collection in your postman software and you should be able to see the API methods under Collections tab as displayed in the image below:
Please look at the how to use page in this document in order to understand the usage of the API plugin.