You can sign-up for the NoteCaddy beta here. To sign-up, you will need your registered email address and registration code as show in PokerTracker under Help -> About.

NoteCaddy does not need to be downloaded.  Once you sign-up for NoteCaddy, it is linked to your PokerTracker 4 registration code and the next time that you start PokerTracker, NoteCaddy will download, install and run the setup wizard.

PokerTracker 4 Hud displays a popup showing sample NoteCaddy data in any profiles which haven't been edited.  If you've made changes to your Hud and you're not seeing the NoteCaddy popup you can create a new profile to see it.
To do this go to Hud > Edit Hud Profiles and create a new profile via the Options button menu, then use Replace Current With Default option from the same menu to set it to the default, and click OK without making any other changes.
You won't see the NoteCaddy popup in the editor, but you will have a new tab in your popup in the Hud.

PokerTracker 4 Hud displays a popup showing sample NoteCaddy data in any profiles which haven't been edited.  If you want to hide this popup tab go to Hud > Edit Hud Profiles and select the profile that you're using.  Click OK to close the window again.

NoteCaddy can be configured to use a different number of threads depending on the balance you prefer between speed and performance. 
To reduce the CPU load on your computer reduce the number of threads which NoteCaddy uses on the 'Creating Notes' pane.
Using more threads can be useful when you have a large backlog of hands to process (if you reset and recreate the notes for example) but in normal use using fewer threads will mean that NoteCaddy uses less processing power, and it will keep up with your live play.