My following reply is for clarity in case anyone else sees this thread. The OP was interpreting this incorrectly. There is in-fact an include/ folder within the project's root folder. The inc/ folder the OP is referencing is within the setup/ folder within the project root. In this case all the OP needed to do was follow the on screen instructions:
Rename the sample file include/ost-sampleconfig.php to ost-config.php and click continue below.
So you need to go to the include/ folder, copy ost-sampleconfig.php to ost-config.php, set the file's permissions to 0666, and click Continue within the installer.
Cheers.