How To: Show a Visual Basic 6 modeless form in ArcMap
Modeless dialogs implemented in Visual Basic 6 (VB6) no longer work the same within ArcMap 9.x as they did within ArcMap 8.x. A more flexible alternative is to create a dockable window.
If a modeless dialog must be used, ESRI Development Network (EDN) has information and sample code on ways to do this.
Please visit the EDN documentation for VB6 Modeless Dialogs here:
VB6 Modeless Dialogs