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just make sure that the file is writable. chmod the file using chmod setting 777 if you are using unix, right click on the mainfile.php file and uncheck the read only checkbox. Then re-load the install page you are viewing, and restart the install.
Basically, its just telling you that the mainfile.php file has to be writable by the server. Once the install is finished, change it back to read only checking the read only box if you are in windows, or chmod it to 755 if you are running unix, or if your host is running unix.