pot player

using an hp elite 8000 cmt/ win 7, how do I enable the rear line in(blue port) and then record that signal to the potplayer? as far as I can tell the port is active and yet there is an error displayed in the potplayer, "there is no audio present at the port". This is not true as the recording devices screen shows an active port. I CAN record radio from my app and youtube videos by copy and paste into the player. only the line in does not work. vlc gives the same error. solution please. the signal comes from the set top cable box. is there a setting I am missing? how to correct?