I think this is what you are looking for, but let me explain where the code must reside.
Since you want the time to happen by a click event you must put the code in the
Worksheet SelectionChange event in the worksheet itself and not just create a macro.
You stated you are using Office 2007 so here is what you do.
1) Click on the Developer Tab.
2) Click on the Visual Basic icon at the top left of the ribbon.
3) Now on the left pane window, you will see a folder called Microsoft Excel Objects that should be expanded already with the worksheet names of your workbook.
4) Double click on the worksheet where you want the time to appear.
5) Now you will see two drop down boxes; on the left you will see (General), and on the right you will see (Declarations).
6) Click on the left drop down and change (General) to Worksheet.
7) Now you will see Private Sub Worksheet_SelectionChange(ByVal Target As Range)
8) This is the event you need for the mouse click. You will only have to click the mouse once.
9) Copy and paste this code between Private Sub Worksheet_SelectionChange(ByVal Target As Range)
and End Sub.
If Target.Address = ActiveCell.Address Then
Target = Format(Now, "ttttt")
10) Close the Visual Basic Editor and give it a try.