If you have questions about extension and user DLLs, be sure to read technical notes 11 and 33. Volume 2 of the documentation has more info too. Finally, try searching on the sample names dllhusk (extension DLL) and dlltrace (user DLL) in books on-line.
10.1. Do I need a CWinApp object in a DLL?
10.2. How should I define the WEP in a MFC DLL?
10.3. How do I build an 'Extension DLL'?
10.4. How can I manage resources in a 'Resource only DLL' and still benefit from ClassWizard?
10.5. I am having [some problem] with exports/imports and extension DLLs.