If you would like the word search appears in the textbox and disappears when the user clicks on the box, read on, this is the way it was implemented by one of our customers:

This example shows how to do it for the skin object in this directory:  

dnnroot/DesktopModules\XSSearchInputSkinObject/XSSearchInputSkinObject.ascx

The same will work for the module version of the search box in this directory:

dnnroot/DesktopModules/XSSearchInput/XSSearchInput.ascx

Here is  the skin object modification: modify this textbox:

<asp:textbox id="txtSearch" runat="server" Wrap="False" Width="100px" maxlength="200" cssclass="NormalTextBox"></asp:textbox>

You should modify it to look like this:

<asp:textbox id="txtSearch" runat="server" Wrap="False" Width="100px" maxlength="200" value="Search" cssclass="NormalTextBox" onfocus="this.value='';" onBlur="if(this.value==''){this.value='Search'};"></asp:textbox>