Please note that while you can add NFT's as custom tokens in the extension, you will not be able to see them natively in the UI.
MetaMask mobile displays NFT's under Collectibles. Please remember that some NFT's will not show up on the mobile app under Collectibles, or may not show up correctly.
We pull data from opensea.io, it's our 3rd party repository for all NFTs. A collectible ID is a unique identifier since no two NFTs are alike. Please try finding the ID with the app where you purchased it.
Add the NFT to your MetaMask wallet. We recommend you do so on the MetaMask mobile app, because NFTs are not currently visible on the browser extension.
Make sure you’re viewing the account/address that the NFT was sent to
Select Collectibles → Add Collectibles (If you do not already see your collectibles)
Enter the NFT address and collectible ID and your NFT will appear under the Collectibles tab on your MetaMask wallet
If you are unable to view them in your wallet, try disabling 3rd party browser extensions on your browser except for MetaMask.