To compile a C++ application using gtkmm in Code::Blocks, do the following steps:

1. Open Project > Build Options…
2. Select your project.
3. On the Compiler settings tab, select Other options and add this to the field:
   `pkg-config gtkmm-3.0 –cflags`
4. Select the Linker settings tab and type the following line in the Other linker options text area:
   `pkg-config gtkmm-3.0 –libs`