Aquí hablan más sobre el problema:
http://support.embarcadero.com/article/37107
Para BDS2006:
Cita:
C++ Notes
If you are creating a C++ Package that contains a Delphi unit that
uses the DesignIntf and DesignEditors tools, you must choose
Project|Options|Pascal Compiler|Other Options and then add
-LUDesignIDE for the compiler to work correctly.
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Para CB2007/RAD Studio 2007:
Cita:
Project|Options|Pascal Compiler|Other Options|"Use these packages when
compiling", then add DesignIDE in the combobox.
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El problema es que yo uso Borland C++ Builder 6 y no veo la opción Project|Options|Pascal Compiler|Other Options.