To use The Giant piano patches in Sunday Keys, first you'll need to download the free Kontakt 6 Player here.
Double check that you have The Giant downloaded and installed inside Native Access:
After that, you'll want to go to The Giant patch and in the channel strip you can go to Input > AU Instruments > Native Instruments > Kontakt and add that:
When you click to open the plugin you should see this:
The Giant might not be loaded in the right side of the screen at first. I needed to click on "Instruments" and then choose The Giant.nki and it loaded in on the right. After that then the patch should be playable:
You'll need to do this for each patch and then save and export to write over the original file and then MainStage should remember it.