The mouse wheel does not work in the Visual Basic 6.0 IDE because that IDE does not have built-in support for scrolling by using the mouse wheel. This behavior is by design. Mouse wheel support in Visual Basic 6.0 is a function of the mouse driver. The WM_MOUSEWHEEL message is sent to the Focus window when you rotate the mouse wheel. Because the Visual Basic 6.0 IDE does not have built-in support for scrolling by using the mouse wheel, the IDE ignores the WM_MOUSEWHEEL message. However, IntelliPoint software version 4.12 provides mouse wheel support and converts the WM_MOUSEWHEEL message to WM_SCROLL. IntelliPoint software version 4.9 and later versions do not have this feature. Therefore, if you want to use the mouse wheel to scroll in the Visual Basic 6.0 IDE, you must use IntelliPoint software version 4.12. An alternative is to download, install, and load the VB6 Mouse Wheel add-in DLL from Microsoft's Web site.
For more information regarding this topic, as well as details regarding how to add support for the mouse wheel in the Visual Basic 6.0 IDE, refer to this article on Microsoft's Web site.