In-App purchases are only available for macOS 10.13 or higher and due to Apple having 2 separate App Stores (macOS App Store & iOS App Store) we can't transfer purchases between the 2 platforms.
To purchase the Blues Styles, click on Store in iReal Pro:


Once you're in the Store, you can select the extra features that you would like to purchase.
You can listen to some samples before you make the In-App purchase.

To restore in-app purchases on another Mac:
If you have already purchased the In-app, you will not be charged again to download it to your other Apple computer.
Mac Store version: click on Store -> Restore Purchases.
You have to be logged in with the Apple ID that you used for the original purchase.
Direct download: click on Buy Blues -> click on Enter license at the bottom right corner and copy/paste your license.

The PURCHASED button will indicate that the in-app was purchased.

The Blues Styles will appear in your accompaniment style list under Blues.

If you still don't see it, quit then reopen iReal Pro.