In excel VBA, call the forms and process functions in the DLL written in VB6

I used VB6 Enterprise Edition to encapsulate a DLL, which has four VB forms: form1, form2, form3, form4, in VBE of excel 2003, class modules and standard modules in DLL are successfully called with VBA code, but VB forms cannot be called or displayed !!! How should I call it? To display ???
The calling process is as follows: Open the excel worksheet and display form1 (Judge and start form4 when form1 is started); click the two buttons in form1 to call form2 and form3 respectively; click the form2 and form3 buttons to execute the sub in the class module.