If you find any lines in that log file saying something about 'no valid sudoers' being found, you have a problem with your system that is interfering with the installation.
Next, search for 'sudoers' (without the quotes). Double-click that install.log file to open it in the Console application. In the folder that opens, an install.log file should be selected. In the Finder, choose Go to Folder from the Go menu, then enter the following path into the window that opens: