I don't know if Plex media server has a built-in web server. You can't surf to a PC if you don't have a web server running on it.
If Plex does have a webserver then you need to go to
http://IP.of.PC:1723/ in a web browser.
If you just want to see files on the computer you need to use an app that supports SMB shares, share something on the PC, then configure the app as required. There is no standard format for that.