I found this problem once on my current installation ,this differed with running steam once after a reboot or something ,but idon't exactly remember that to well
what you can do as an alternitave though is edit the playlist.txt and playlist.def files and put the mp3 file location in those documents (with notepad of course). You have to do it like this to , you can't just add it in as (example :d:/*folder name*/*filename*) , it would look something like this when you add it in : C:\games\HALF-L~1\esf\mp3\01-satellite.mp3.
Hope that solves your problem ,as when the playlist editor doesn't work that's my method of adding songs