Problem scenario
You installed the MetaMask extension on a web browser. You cannot seem to locate it or open it. How do you use MetaMask now?
Solution
Click on the extensions icon of your web browser. If you click on extensions or are viewing your browser's extensions and you do not see it, go to Extensions, and choose the allow option for "Run in Private Windows." This should make it visible for you (even if you were not necessarily opening a private window).