If you want to replace the SharePoint InputFormTextBox on a custom page or control with the Telerik RadEditor you will the MOSS version of it. The following lines of code will do (this is for version 5.2.2 build 826.20):
<%@ Register TagPrefix="telerik" Namespace="Telerik.SharePoint"
Assembly="RadEditorSharePoint, Version=5.2.2.0, Culture=neutral,
PublicKeyToken=1f131a624888eeed" %>
<%@ Register TagPrefix="telerik2" Namespace="Telerik.Web.UI"
Assembly="Telerik.Web.UI, Version=2008.2.826.20, Culture=neutral,
PublicKeyToken=121fae78165ba3d4" %>
<telerik:MOSSRadEditor ID="radEditor1" runat="server"
Width="450px" Height="150px"
ToolsFile="/_wpresources/[...]/toolsfile.xml" />
You only have to make sure to use the editors Content property instead of the Text property when you want to assign text to editor.