I have a os x yosemite 10.10.5 and I was wondering how do you download plugins like noise gate on Macbook?

This page in the manual is the starting point for downloading and installing plugins.

This page has detailed instructions, and a list of downloadable Nyquist plugins.

You don’t say what version of Audacity you are using. We recommend the latest version, available here.

– Bill

I am using the 2.2.0 version