I don't see it in the layout.,aster file you posted. It looks like this:
<mp:AddThisButton ID="at1" runat="server"
Text="Share This Using Popular Bookmarking Services"
However, be aware that the blog has its own add this buttons and doesn't use the one from layout.master. As long as AccountId="" and if you don't enter a setting for Add This in site settings > API keys nor in the blog module settings, add this button will not display even if it is present, because it is designed not to render if the account is blank.
Hope it helps,