Currently Being ModeratedNov 13, 2012 7:53 AM (in response to Nalimanestianu)
I'm assuming you've downloaded mountain lion through the app store? When the download completes, you'll get a window popup asking about installing. Have you seen this?
Check and see if your download is in progress. Open the app store >> click purchases at the top. Does it show as downloading? Paused?
If the download completed, and you didn't get an installer popup, you can check for it in your /Applications folder. It'll be named something like "install os x mountain lion". If it's there, double click it.