On
http://181.fm click on the "Winamp/Itunes" link on the top right, then choose among the links the one you want and copy it to mplayer/playlist. Alternatively, from the url you can see that the name of the station is 181-oldschool, and the pls files url format is
http://www.181.fm/winamp.pls?station=STATION, so just replace STATION by 181-oldschool.