Seems Watcom use H folder for include files. Check TheIde configuration to see if there is no reference to that path.
Or if you have used Watcom compiler to build examples, set in Build Methods.. MinGW as default. However, why do you use Watcom Complier? It is not good at all, and miss more languages features. I know it from when i used it on Windows, now on Linux gcc(g++) is the best. So using MinGW is the best (second) choice. First one is M$ Compiler, which is optimized for exe format only. The MinGW comes with Upp or you can download latest version from it site and also MS give a free version of his compiler, so you can use booth.