The ShowEvent is specified in teh markup declaration (or dynamically in code behind). However, it is not directly related to the OnAjaxUpdate event and should not be set there. The OnAjaxUpdate event is used only to populate the content of the tooltip. This being said, in ordre to add tooltips to your textbox, you should go through the following steps:
1) Add the textboxes to the TargetControls collection - you can do this on teh server or in the markup.
2) When you add each textbox, add a Value - it can be the string you want to show or a primary key value which should be used to extract information from a DB.
3) In the OnAjaxUpdate (you can have the same handler for both if the tooltips should have the same information) you put the content of the toooltip.
This approach is shown in all the LOD demos, including the one you have linked. The only difference between the demo you have examined and your setup is that you do not have an event as ItemDataBound where to attach tooltips but you can simply iterate through the textboxes or even easier - set them in the markup.
the Telerik team