How do I find Planet IX assets on OpenSea?

They might be hidden. This is a safety precaution from Opensea as airdropped NFTs are by default the hidden section. To unhide them go to your Opensea profile, click on more and then hidden. Use the search field to quicker find the NFT you are looking for. Click on the three (3) dots and unhide. Hidden tab picture is in the Picture folder.