Q: have a new mac but no music when i sing into my iTunes account

The iTunes application saves your iTunes Store purchases--as well as any music, audiobooks, or videos you import--to your computer's hard disk, not the iTunes application itself.

Fortunately, if you didn't back up your files(which you should do regularly) and don't have access to your old computer containing your purchases, the iTunes Store gives you the ability to download your past purchases without incurring additional charges as long as the items are on your Purchased page. You may be able to use this feature to download your purchases.

To learn how to take advantage of this feature, visit:

Downloading past purchases from the App Store, iBookstore, and iTunes Store

http://support.apple.com/kb/HT2519

If you have items on your iPhone, iPad, or iPod, you can also use iTunes to transfer them to your computer. This article will walk you through that:

iTunes Store: Transferring purchases from your iPhone, iPad, or iPod to a computer

http://support.apple.com/kb/HT1848

The music does not magically appear.

It will only be where you put it.

Copy it from your old computer, or your backup copy of your old computer, to your new one.