If you want to add any file in a project in Creator which is at a path outside .cydsn project folder then add the path of that file in Build settings>Compiler>General>Additional Include Directories.
Follow these steps then:
>>Right click the main.h file with question mark and exclude it
>>Next, include it again by right clicking the Header files and choosing Add >Existing Item option.
I did so and I was able to clear that build error associated with main.h being missing.
But certain other errors are there in the project which are related to undeclared variables.