They are contained in your iTunes Library which you need to copy from your old computer's /Home/Music/ folder and install on the new computer in the same location. Also copy the com.apple.iTunes.plist preference file from /Home/Library/Preferences/. The Library file is invisible, so see the following:
Exposing the /Home/Library/ Folder
Pick one of the following methods:
A. This method will make the folder visible permanently. Open the Terminal application in your Utilities folder and paste the following at the command prompt:
chflags nohidden ~/Library
Press RETURN.
B. Click on the Desktop, press the OPTION (⌥) button, select Library from the Finder's Go menu.
C. Select Go To Folder from the Finder's Go menu. Paste the following in the path field:
~/Library
Press the Go button.