With new iphone ios 6 we can't add music by copy and paste but there are a way that you can add music :

1- Update itunes to at least 10.6.3

2- Open itunes and connect your iphone create new playlist

3- In your new playlist add the music that you want to copy

4- Then go to your iphone and click on music and check (sync music) and in playlist check your new playlist

Now it will sync your music . you can add music by adding it to your your playlist then sync to your iphone .

I hope this useful :)