how do i authorise a new computer

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: