SSO Sampling (sample subset optimization) is free and open-source tool implementing a hybrid algorithm which uses internal cross-validation and sample subset optimization technique to rank samples from the majority class.
The top ranked samples are combined with samples from the minority class to create an optimally balanced dataset.
Detailed instructions on how to install and use the SSO Sampling utility on your Mac are available HERE.
SSO Sampling is a cross-platform utility capable of running on any operating system that comes with Java support (e.g. Mac OS X, Windows, Linux).