The polkadot.js webapp and browser extension only show your mnemonic seed once: during the initial creation of your account. 

If you miss taking note of it, or if you misplaced your seed, there is no way to view it again at a later stage. While you can always restore your account from your JSON file, you won't be able to see your mnemonic seed again.

The only workaround for this would be to 
  1. Create a new DOT account
  2. Take note of the mnemonic seed and save your new JSON file in a safe and secure place
  3. Send your funds from your current account to the new one. (You will have to switch the "keep alive checks" off in the transfer screen in order to send all of your funds out.)