When I type it here :-
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder2" Runat="Server">
<link href="Skins/SkinCustom/Editor.Default.css" rel="stylesheet" type="text/css" />
</asp:Content>
Its says Element Link cannot be nested within div...how am I supposed to link my css files ??
If your master page does not provide a content placeholder in the <head> section (ed: and you can't edit it), you may be out of luck.
Have a look at the master page source and see what other content placeholders are available.
Add a ContentPlaceHolder in your master's head tag:
<head>
<asp:ContentPlaceHolder ID="head" runat="server"></asp:ContentPlaceHolder>
</head>
Then in your content page:
<asp:Content ID="Content1" ContentPlaceHolderID="head" Runat="Server">
<link href="Skins/SkinCustom/Editor.Default.css" rel="stylesheet" type="text/css" />
</asp:Content>