By default, Windows hides a lot of files and folders including many of the support files Outlook uses. In order to find it using Search, you need to use the Advanced search option to search all hidden files and folders and all system files.
Although Outlook's PST file is stored in a folder that is marked hidden, when Outlook is open, its easy to get to the folder where the PST file is kept. Once the location is open in Windows Explorer, you can close Outlook, if needed. (Outlook needs to be closed to move, copy, or delete data files that are in the profile.)
Windows 7 and Vista users: To access the default data file location when Outlook is closed, press Windows key + R to open the Run command and type or paste the following line in the dialog and press Enter.
Note: Outlook 2010 uses a folder under My Documents as the default data file folder for .pst files used by POP3 accounts and pst files you create. The default location for IMAP and Sharepoint .pst files as well as Hotmail Connector and Exchange mailbox .ost files remains in %localappdata% path.
Find Outlook Data Files
Where are Outlook's Files? (Outlook-tips.net)