Both RadToolTipManager and RadToolTip controls can be used for tooltipifing. However, there are some significant differences in both implementations and usage and you should decide which approach to choose depending on your particular scenario.
When using the RadToolTipManager control you should iterate through the controls you want to tooltipify and add them to the manager's TargetControls collection. This requires more code than the scenario which uses the RadToolTip control but its main advantage is that the tooltips' content is loaded through AJAX, in the RadToolTipManager's OnAjaxUpdate eventhandler. Furthermore, this approach is suitable for scenarios with WebServices. You can find a similar, working demo here.
It is more comfortable to use the RadToolTip control when you want to tooltipify a single element. The RadToolTip control can also be included in Item Templates and its content can be set through declarative binding. This requires little code, in fact just declarative setting of the Text property. In this case, the content is not updated through AJAX, it is constant. You can find a similar, working demo here.
In case you still experience problems, please open a new support ticket and attach a fully working project with a detailed explanation of your scenario and the experienced problem.
the Telerik team