I'm not aware that any true menus on a VB6 for (even non-MDI) have a tool-tip property; hence, neither does the MDIForm when one its child forms has its menu displayed.
However, there are a few controls that can be put on the MDI parent form. The most useful of these is probably the PictureBox, into which you can then place any type of control, such as CommandButtons. It must be "docked" to either the top or bottom of the MDIForm, using the PictureBox.Align property. Controls within the PictureBox can be located any way you wish, and can have their normal ToolTip properties set.
Another MIDForm-usable control that I've had good luck with is the ToolBar control, which comes in as part of the CommonControls component. Like the PictureBox, it has a .Align property to set it to the top or bottom of the MDIForm. You can put any reasonable number of "buttons" on it, and have reasonable control over their appearance (graphics, etc.); apropos the OP's query, you can assign a ToolTip to each button. It wouldn't be difficult to have the buttons change depending upon the currently-active form, just as the MDI menus normally do.
Thank you for the feedback! I'll try using both and see which one best adapts to the requirement.
This is a good approach, and undoubtedly the easiest, if you are willing to forgo a true menu. Another option would be a 3rd party menu control that supports tool tips. Yet another option would be to subclass the main form, and although the hardest approach, would possibly allow you to achieve what you need.