You should firstly install the RadEditor for SharePoint by following the instructions provided in this article: Installing RadEditor & RadGrid Web Parts on SharePoint 2010
You can download the installation of RadEditor for SharePoint 2010 from the following location: http://www.telerik.com/account/your-products/trial-product-versions/trial-single-download.aspx?pmvid=2665&pid=561.
You can download the automatic SharePoint 2010 Web Parts MSI installation as well as the manual SharePoint 2010 Web Parts WSP archive.
Once you install the editor, you can follow the instruction in this article: Using RadEditor in List Items
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