MacPorts, by default, installs into
/opt/local/, judging by the screen shot ack should be in
/opt/local/perl/p5-app-ack/ if you used the default install path. Make sure that path is in your $PATH environment variable.
To add it to your path edit your shell config file (probably ~/.profile or ~/.bashrc) and add or modify the following line.
I don't use MacPorts for ack, I install it manually with the following command from the ack install page.
curl http://betterthangrep.com/ack-standalone > ~/bin/ack && chmod 0755 !#:3
This will install ack into the
$HOME/bin/ folder as a single large perl file and I did not have to modify my $PATH variable.