Here's how to do it:
(1) Create a new file (File | New... from the menu bar)
(2) Select 'Empty Project'
(3) Create a New Track, and make sure it's a Software Instrument, as follows:
(4) Assuming you haven't customised your Logic layout too radically, on the left-hand side you'll see the Arrange channel strips like this:
(5) On the left channel strip, just below the 'I/O' label is a blank rectangle. Hold your mouse down and select 'AU Instruments | Garritan | ARIA Player | Stereo' like this:
(6) You'll then see the Garritan ARIA Player pop-up. Select an instrument as you would in ARIA i.e. click your mouse on an empty instrument bank and hold your mouse down to navigate to an instrument of your choice, like this:
(7) Close the ARIA Player and you're back in Logic and your instrument is available to you!
(8) If you want to change your instrument go back to your Arrange channel strips and double click the rectangle that was blank earlier and now says 'ARIA Player':
