Is there any reason you're not using a table layout? Its what I've found the easiest way to display the text editor where I want it. If you're opposed to a table layout, you will have to float the containing div that the editor will get rendered wrapped in.
<table class="dnnFormItem">
<tr>
<td><dnn:label id="plField" runat="server" text="SendTo" helptext="Enter a
value" controlname="txtField" /></td>
<td><asp:textbox id="txtField" runat="server" maxlength="255" /></td>
</tr>
<tr>
<td><dnn:label id="plField2" runat="server" text="SendFrom" helptext="Enter a
value" controlname="txtField" /></td>
<td><asp:textbox id="txtField2" runat="server" maxlength="255" /></td>
</tr>
<tr>
<td><dnn:label id="plField3" runat="server" text="Subject" helptext="Enter a
value" controlname="txtField" /></td>
<td><asp:textbox id="txtField3" runat="server" maxlength="255" /></td>
</tr>
<tr>
<td><dnn:label id="plField4" runat="server" helptext="Enter a value"
controlname="txtField" /></td>
<td><dnn:TextEditor ID="txtField4" runat="server" /></td>
</tr>
</table>