You can only put .oggs in game so you need to convert your .mp3, put this in your description.ext
class CfgMusic
{
tracks[]={yourmusicname};
class yourmusicname
{
name = "whatever";
sound[] = {\music\yourmusicname.ogg, db+10, 1.0};
};
};
For your other prob just single click your description.ext.txt and press F2 to rename it then delete the .txt bit.