To add a new definition to a Xpresso-IDE project, open the project's settings menus by right-clicking on the project title in the Project browser and selecting Properties.

In the left menu, select C/C++ Build with the sub-option Settings. In the right view in the Tool Settings list, select Symbols. Add new symbols by clicking on the green plus icon. Definitions that have a value should have the value assigned with an '=' statement after the definition.