The InsertTable tool is disabled by default. nyway, you can easily turn it on. Open the EditorToolsFile.xml file located in the ~/Sitefinity/Admin/ControlTemplates/ folder and add the following:
| <tool name="InsertTable" />
Also, here is a reference to an useful online article
that describes how to use the InsertTable
Do let us know if you need any further assistance.
the Telerik team