You need to go to the little square in the upper left-hand corner, and if you click on that you will see an option for Store. Click on Store, and then you will see an option to Authorize the computer (you need to be signed on to iTunes to do the Authorization). Once you have authorized, you will be able to see all of your purchased music as follows:
Go to the iTunes STORE, and under Quick Links, select Purchased. Once you select that, then you can select the Music tab at the top, and then select "All Songs" and all of your purchased music will display. If it has a cloud next to it, it can be downloaded to the computer. If you want to download all of the music, you can click on "Download All" at the bottom right.
There's a couple of ways to get through to the authorisation controls in the 11.0.x versions.
The control is still in the Store menu, but first (if you're using iTunes versions 11.0.x) you might need to bring up the menu bar to see the Store menu.
If you're using 11.0.x, click on the wee boxy icon up in the top-left corner of your iTunes to see the "Show Menu Bar" control, as per the following screenshot:
Then you'll find the control in the Store menu:
Alternatively, if you don't want to bring up the menu bar, it's still possible to get into the authorise controls via nested menus accessible from the wee boxy icon. Here's a screenshot of where to find them: