mIRC 6.35
The latest release of mIRC is 7.19 which has not been fully tested with QuackScript. There shouldn't be any issues with it, but it's possible some of the features may not work quite as expected or there may be bugs. You can certainly try using the script with 7.1x but please don't complain if there are issues.
The last version of mIRC that has been thoroughly tested is 6.35. You can download mIRC 6.35 from FileHippo.
QuackScript is considered unsupported from December 2010. I just don't have the time to make sure everything is still working whenever a new version of mIRC comes out or to add new features to the script.