i did this once before, but have since forgotten how. i seem to remember having the ack.pl file in an "opt" directory close to root level on my Mac. i also remember that i will need to update my path settings in my ~/.profile or ~/.bash_profile, etc. I hope someone can fill in the blanks with some detailed instructions on how to do this properly.
OK - found the answer myself. Here's how i did it on OS X 10.10 Yosemite (no "opt" directory, no messing w/ editing my $PATH in /.profile, etc.)...
1 - set permissions on the directory /usr/local/bin to have read/write access for everyone.
(chmod 777 from Terminal - or - command + i from Finder, and set all to "Read & Write")
2 - then run the following command from the Terminal...
curl http://beyondgrep.com/ack-2.14-single-file > /usr/local/bin/ack && chmod 0755 /usr/local/bin/ack
(obviously the path to the file on http://beyondgrep.com will change as they release newer versions, so update this adddress accordingly from information at - http://beyondgrep.com/install/
3 - change read/write permissions on /usr/local/bin back to 755 ("Read & Write" by "system" only, group "wheel" and "everyone" set to "Read only")
and that's it!