This
article from Squarespace support explains how to add code to a Squarespace site. The Slickstream embed code should be added to the Header (i.e. within the <head> tag) of your Squarespace site.
The Slickstream embed code is available in your portal. Sign in, and click on the Embed tab. From there, copy the complete embed code from the textbox in the Embed Code section.
Paste the entire embed code into the <head> of your site.
If you run into any problems with these instructions, don't hesitate to contact us at support@slickstream.com.