Previously, using GNU Unix I had no problem including a text file to supply initial value for a simulation program.
Kdevelop has me confused, the text file is simple ascii, labeled "initial values.inp". Where can I put the file so c++ can read the file?

There's no difference introduced here by KDevelop. If you want to read the file with just "fopen(bla)", you need to put it into your current working directory. Which, in case you start your executable from KDevelop, can be set in the run configuration dialog.

By the way, "GNU" stands for "GNU is not Unix", so "GNU Unix" is somewhat confused