Well, button is on main form and textbox is on subsubform - this will get a bit tricky. Must reference subforms through the containers that hold them. You have to go through two subforms.
However, there is something very wrong with Form1. Could not get any code in the button Click event to work, not even a simple Debug.Print "Test". Deleting and replacing the button didn't help. Had to build a new form. Then this worked:
Private Sub Command0_Click()
Me.Form2.Form.Form3!Textbox0 = 5
End Sub
I normally given container control a name different from the object it holds, like ctrForm2. Then code is:
Private Sub Command0_Click()
Me.ctrForm2.Form.ctrForm3!Textbox0 = 5
End Sub