You must add the ValidateRequest="false" to your Page Directive:

<%@ Page Language="C#" MasterPageFile="~/ftb/MasterPage-FTB.master" AutoEventWireup="true" ValidateRequest="false" CodeFile="freetextboxbasics.aspx.cs" Inherits="ftb_freetextboxbasics" Title="AADeveloper: Free Textbox: Basics" %>

You must add the Registry under the Page Directive:

<%@ Register assembly="FreeTextBox" namespace="FreeTextBoxControls" tagprefix="FTB" %>

You add the Free Textbox Module:

<FTB:FreeTextBox ID="FreeTextBox1" runat="server" AllowHtmlMode="False"
AssemblyResourceHandlerPath="" AutoConfigure=""
AutoGenerateToolbarsFromString="True" AutoHideToolbar="True"
AutoParseStyles="True" BackColor="158, 190, 245" BaseUrl=""
BreakMode="Paragraph" ButtonDownImage="False" ButtonFileExtention="gif"
ButtonFolder="Images" ButtonHeight="20" ButtonImagesLocation="InternalResource"
ButtonOverImage="False" ButtonPath="" ButtonSet="Office2003" ButtonWidth="21"
ClientSideTextChanged="" ConvertHtmlSymbolsToHtmlCodes="False"
DesignModeBodyTagCssClass="" DesignModeCss="" DisableIEBackButton="False"
DownLevelCols="50" DownLevelMessage="" DownLevelMode="TextArea"
DownLevelRows="10" EditorBorderColorDark="Gray" EditorBorderColorLight="Gray"
EnableHtmlMode="True" EnableSsl="False" EnableToolbars="True" Focus="False"
FormatHtmlTagsToXhtml="True" GutterBackColor="129, 169, 226"
GutterBorderColorDark="Gray" GutterBorderColorLight="White" Height="450px"
HelperFilesParameters="" HelperFilesPath="" HtmlModeCss=""
HtmlModeDefaultsToMonoSpaceFont="True"
ImageGalleryPath="~/ftb/images/"
ImageGalleryUrl="ftb.imagegallery.aspx?rif={0}&amp;cif={0}"
InstallationErrorMessage="InlineMessage" JavaScriptLocation="InternalResource"
Language="en-US" PasteMode="Default" ReadOnly="False"
RemoveScriptNameFromBookmarks="True" RemoveServerNameFromUrls="True"
RenderMode="NotSet" ScriptMode="External" ShowTagPath="False" SslUrl="/."
StartMode="DesignMode" StripAllScripting="False"
SupportFolder="/aspnet_client/FreeTextBox/" TabIndex="-1"
TabMode="InsertSpaces" Text='<%# Bind("WebContent") %>' TextDirection="LeftToRight"
ToolbarBackColor
="Transparent" ToolbarBackgroundImage="True"
ToolbarImagesLocation="InternalResource"
ToolbarLayout
="ParagraphMenu,FontFacesMenu,FontSizesMenu,FontForeColorsMenu,FontForeColorPicker,
FontBackColorsMenu,FontBackColorPicker|Bold,Italic,Underline,Strikethrough,Superscript,Subscript,RemoveFormat
|JustifyLeft,JustifyRight,JustifyCenter,JustifyFull;BulletedList,NumberedList,Indent,Outdent;CreateLink,Unlink,
InsertImage,InsertDIV|Cut,Copy,Paste,Delete;Undo,Redo,Print,Save|SymbolsMenu,InsertDiv,StylesMenu,
InsertHtmlMenu|InsertRule,InsertDate,InsertTime|InsertTable,EditTable;InsertTableRowAfter,InsertTableRowBefore,
DeleteTableRow;InsertTableColumnAfter,InsertTableColumnBefore,DeleteTableColumn|InsertForm,InsertTextBox,
InsertTextArea,InsertRadioButton,InsertCheckBox,InsertDropDownList,InsertButton|InsertDiv,EditStyle,
InsertImageFromGallery,Preview,SelectAll,WordClean,NetSpell"

ToolbarStyleConfiguration="Office2003" UpdateToolbar="True"
UseToolbarBackGroundImage="True" Width="712px">
</FTB:FreeTextBox>

Add the dll and js files to the bin folder in your project:
Free Textbox: AADeveloper: Bin