To stop users from leaving the form, on the Before_Update of the form
If isnull(me.NameOfDropdown) then
cancel = True
End if
To stop editing the data before this is filled in, you need initially to disable all of your editable controls except the dropdown then on the After_Update of the Dropdown, check if it is null and if not, re-enable all of the controls. for example
Form_Current (got it right this time Rich

)
if isnull(me.nameofDropdown) then
Call LockAllControls(me)
me.NameOfDropdown.locked = false
else Call EnableAllControls(Me)
end if
DropdownBox After_Update
Form_Current
In a seperate module
Public Sub EnableAllControls(Frm As Form)
Dim ctl As Control
For Each ctl In Frm.Controls
Select Case ctl.ControlType
Case acTextBox, acComboBox
ctl.Enabled = True
ctl.Locked = False
ctl.BackStyle = 1
End Select
Next ctl
End Sub
Public Sub LockAllControls(Frm As Form)
Dim ctl As Control
For Each ctl In Frm.Controls
Select Case ctl.ControlType
Case acTextBox, acComboBox 'ie Editable Controls
ctl.Enabled = True
ctl.Locked = True
ctl.BackStyle = 0
End Select
Next ctl
End Sub