You must use CSS to align the text box. The reason your code above does not work is because by default the width of the div is the same as in the container, so in your example it is clicked below.
.
<td colspan="2" class="cell">
<asp:Label ID="Label6" runat="server" Text="Label"></asp:Label>
<asp:TextBox ID="TextBox3" runat="server" CssClass="righttextbox"></asp:TextBox>
</td>
CSS:
.cell
{
text-align:left;
}
.righttextbox
{
float:right;
}