you could use the afterupdate event of the combo box to populate the text
box.  Something like,

If isnull(Me.ComboboxName)=False then
   Me.TextboxName = Me.ComboboxName.Column(1)
End if

An alternative would be to use dlookup to get the name based on the id.

A third selection, aside from Daniel's two, is to set the combo's Bound Column
property to the one you want stored (the ID); the column widths property can
be set to 0 for the ID if you don't want it visible.

Or, you can put a textbox on the form with a control source

=comboboxname.Column(n)

where n is the *zero based* index of the field you want to display - (1) to
show the second column.

            John W. Vinson [MVP]

The two answers already provided will display the employee name, but if you
want the value to save in the field you can do something like this:

Private Sub cboEmpID_BeforeUpdate(Cancel As Integer)
Set rs = CurrentProject.Connection.Execute("select [EmpName] from Topics
where EmpID = '" & Me.EmpID.Value & "'")
Me.[EmpName] = rs("EmpName")
Set rs = Nothing
End Sub

End Sub