For me I use:

In the search dialog:

     answers:0 [tag]

or my favorite that HopelessN00b helped me with:

     answers:0 intags:mine

And THEN click the `newest` tab


----------

This assumes by "unanswered" you literally mean 0 answers so far, if you want "unanswered" meaning that the OP hasn't accepted an answer yet you would use:

    hasaccepted:no [tag]

or

    hasaccepted:no intags:mine

and then click the `newest` tab