First, log in to your account. Once you're logged in, you would be taken to the exchange page and you see a pop-up window inviting you to connect your wallet.
If you don't see it, you can easily find this window in the top right corner "Connect Wallet" button.
Click the MetaMask icon (first from the left).
Click the "Connect MetaMask" button.
Log in to your MetaMask (This step would be skipped if you're already logged in.)
Click the "Connect" button.
Congrats! Your MetaMask is now connected.