After you have successfully signed into your Musicnotes.com account, whether or not you immediately choose to download your music, you will be shown the main viewing window.
In this window you will see the "Musicnotes Cloud" and "Computer" filters on the top left. To view the music you currently have downloaded to your Mac, use the "Computer" filter.
You can use the "Musicnotes Cloud" filter option to view all of your purchased music and select which titles to download to your computer. Just click the gray note icon next to the name of the piece to download it.
In both filters you may remove a piece of music from your computer by clicking the green note icon next to it's name.
If you made a recent purchase which is not in the "Musicnotes Cloud" list, hit the "Refresh List" button and it should appear. If you still do not see your purchase, double check that you are signed into the same account in which you made the purchase.
Next to the refresh icon is a menu button which displays further options. These menu options will let you "mass edit" your purchased music and download all of your purchases at once, or remove everything that is currently downloaded.
If you wish to download a different key of a piece you have already imported, please see additional info in Transposing Your Sheet Music
You can double-click a row to open a file into its own viewer window. (Note: If you double-click a piece that you have not yet downloaded, allow for a longer load time as it downloads before opening.) For more information on the Viewer Window, see Viewing your Purchased Sheet Music.