The PAR window will be automatically opened and started by ezNZB when it has finished downloading PAR files – if there are any present in your NZB file. The PAR files are used to check for errors in the other files downloaded, and then to fix any errors that are found. This is done because the Usenet newsgroups don’t have built-in error checking.
When ezNZB opens the PAR window, you don’t need to click the “Select PAR File” button. That button is there so that you can use the PAR window in “stand-alone” mode to process PAR files that you have downloaded with other programs. To do that, you would go onto ezNZB’s “File” menu, and then to “PAR Decoder” to open the PAR window. Then you could click the “Select PAR File” button, or drag-and-drop a PAR file from your desktop onto the button.