The Salesforce Sidebar is a part of Yesware's Sidebar. The Yesware Sidebar lives within Gmail's taskpane on the right hand side of your inbox. When you click on the blue Yesware icon, you will launch the Yesware Sidebar which will allow you to see the Salesforce Sidebar, Campaigns to-do list, LinkedIn Sales Navigator integration, and Response Insights info.
Because Yesware is nestled in the Gmail sidebar, you must have the Gmail sidebar exposed to access the Yesware Sidebar. If you cannot see the Gmail sidebar, at the bottom right of Gmail, there should be a little tab on which you can click to "show side panel," which should do the trick.