Optimzely is a popular web optimization tool. You'll integrate Optimzely into your storefront by adding their script to the head section of the page.
So, to integrate this into the storefront you'll edit the "document_top.vm" template in the templates section of your storefront host.
Navigate:
3. Edit the "document_top.vm" template and place the Optimizely code within the head tags.
https://help.optimizely.com/hc/en-us/articles/200904580-Frequently-Asked-Questions