You'll need to reinstall pow after updating the POW_DOMAINS env var (see http://pow.cx/manual#section_3.1 for more details).
echo "export POW_DOMAINS=test,local" > ~/.powconfig
curl get.pow.cx | sh
Now you should be able to access your local apps (that are being server by pow) at the .test domain.
See also: basecamp/pow#547