If you haven't copied the music from your old computer or downloaded it from the Purchased area of the iTunes Store, then there is no music on your new Mac. Go to the iTunes Store, log into your account, and click the Purchases link under the Quick Links. From there you should be able to re-download some or all of your purchased content. Note that not all content has been licensed for re-downloading in all countries at this time. You can see what content you can download here:
For full instructions, see:
Any content not purchased from the iTunes Store you'll have to transfer from your old computer or obtain from the original sources.