I have one project that only shows the inst_ref and enumeration types in the Set Type dialog box. None of the base types, e.g. Int, Void, Real, String appear in the list. It appears to be a project level issue but I cannot find a setting.
Am I missing a setting or have a corrupt project? Any ideas on how I might fix it?
That definitely sounds like a corrupt project to me. I suggest you create a new xtUML project, do a quick test (say add a package and a function and then run Set Return Type…) and make sure the type picker shows all the core types. If it does, then export your model from the first project and import it into the new project.