To compile a project that depends on another project, you need to add both projects Studio Workspace, and open them. If in turn the base project depends on another project you need to add it to your Studio WorkSpace and open it as well. If the base project is closed then you get errors when you compile your dependent project.
If someone else developed the project and shared it with you through a version control system, then make sure you checkout all the projects it depends on.
If you import a project that depends on other projects, the exported project file must include the dependencies. For details on how to export a project with dependencies, see Exporting a Project With Dependencies.