Mountain Lion-password prompts
Open Applications > Utilities > Disk Utility.
Select your hard drive in the left column. Most likely it will be named “Macintosh HD.”
Click on “Repair Disk Permissions.”
Close Disk Utility. Check if the problem is still there.
If it is still not resolved, restart and hold down the Command and R keys. You will boot into the Repair Utilities screen. On top, in the Menu Bar click the Utilities item then select Terminal.
In the Terminal window, type reset password and hit Return. The Password reset utility launches, but you’re not going to reset the password.
Instead, click on the icon for your Mac’s hard drive at the top. From the drop-down below it, select the user account where you are having issues.
At the bottom of the window, you’ll see a button labeled ‘Reset Home Directory Permissions and ACLs’. Click the Reset button there.


Source: http://osxdaily.com/2011/11/15/repair-user-permissions-in-mac-os-x-lion/