如果你引用没有错误，页面里也正常添加了些控件。直接在CS代码里FreeTextBox1.Text赋值就可以了。

以前自己的做的记录：

1.FreetextBox.dll拷入bin目录，并添加引用
2.<%@ Register TagPrefix="FTB" Namespace="FreeTextBoxControls" Assembly="FreeTextBox" %>在使用的页面上加上
3.<FTB:FreeTextBox id="FreeTextBox1" runat="Server" />
4.ToolbarStyleConfiguration = Offoce2003/OfficeXP/Office2000/MAC　　皮肤应用
5.<%@ Page language="c#"  ValidateRequest="false"%>　客户端中检测到有潜在危险的
6.完美应用：ImageGalleryPath="~/images"指定文件上传路径，并把ftb.imagegallery.aspx复制到当前文件
<FTB:FreeTextBox id="FreeTextBox1" Focus="true"
            ToolbarStyleConfiguration="office2003"            
            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|Cut,Copy,Paste,Delete;Undo,Redo,Print,Save|
SymbolsMenu,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"
            runat="Server"
            GutterBackColor="red" ImageGalleryPath="~/images"
/>
7.获取值：FreeTextBox1.Text

赋值直接把freetextbox当textbox用就可以了。

this.freetextbox_id.text="aaa";