Thank you so much James,
More details for those who might have the same problem (with the OS X operating system at least) :
It works perfectly as soon as the file tlc_config.h (inside the uploaded Tlc5940 folder) is opened by TexEdit and not by Arduino IDE.(Command +I).
In TextEdit the modification can be made and registered.
Returning to a file openable by Aduino IDE (Command +I again) everything is in order.
marcus