Hopefully you figured this out on your own but for anyone else wondering, register on the site for free, then login, then download the GSAP zip file from Docs > Installation which will automatically include the bonus plugins (including customease). Customease is NOT available on CDN. So you need to download the zip file. If you prefer, you can still use gsap core from CDN, but you'll have to load customease.min.js from your local/site directory.