In Delphi 2009 there is a package, that was installed properly, and works.
However when a code compiles it throws an error
error E2003: E2003 Undeclared identifier: 'TSomething'
Why and how to fix it? (include the .dcu? the .pas?)
The project is build without “build with runtime packages” option.
You probably have forgotten to include the unit that contains the definition of TSomething. Being in a package doesn’t mean you don’t have to use the units explicitly.
Another possibility is that TSomething is defined in the implementation part of a unit in which case it can’t be accessed from outside the unit.