MetaMask browser extension
It's currently not possible to view NFTs in MetaMask Extension. MetaMask recommends you go to the Portfolio Dapp via portfolio.metamask.io, or follow the 'Portfolio site' link in MetaMask, right under the 'Assets' and 'Activity' tabs.
You'll also need to turn on NFT auto-detection and you can follow MetaMask's help center article to get started.
MetaMask Mobile App
MetaMask Mobile App displays NFTs under the NFT tab. We pull our NFT data from IPFS and opensea.io and some NFTs may not show up correctly.
If your NFT doesn't show up automatically, try the following to make it appear:
Find the NFT's address. On OpenSea, for example, on the NFT's page under Details, there is a blue hyperlinked value labeled 'Contract Address'. Click on this to view the contract's address on Etherscan; at the top-left of that page, click on the icon labeled 'Contract', and to the right, the long string of letters and numbers. This is your NFT contract address. Click on the 'copy' icon to the right of the address.
In MetaMask Mobile App, tap on the 'NFTs' tab, scroll down and tap on the '+ ADD NFTs' link. Paste the NFT contract address into the 'Address' box.
Find the NFT's ID. A collectible's ID is a unique identifier since no two NFTs are alike. On OpenSea it is under "Details". Make a note of it, or copy it onto your clipboard.
Paste the ID into the box marked "ID" in MetaMask Mobile, tap on the "ADD" button, and your NFTs should appear under the NFTs tab.