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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