First get whatever Mp3's you want to put on the game, then open up MyDocuments/EAGames/
TheSims2/Music/(whatever RadioStation you want your song on). Then, Drag your MP3 into 
the folder. Now, start Sims 2 then go to options menu, then music settings, then the 
Radio Station, then pick witch songs you want the radio to play. You can now listen 
to your favorite songs in your Sims game.