There are some small differences between the user interface on Sketch for Confluence on Atlassian's Cloud and Server hosting options.

Cloud User's Guide

Once an image has been attached to a page look for the new Sketch link at the top of the published page.

Clicking Sketch opens the image selector menu when you can choose which image on the page you want to edit.

Clicking on the image you want to edit will open the Sketch for Confluence editor.

Tools Guide

To use any tool provided by Sketch simply select the tool from tool panel, located at the top of the Sketch dialog box.

Image Tools

Server and Data Center User's Guide

When editing an image on a Confluence page a new option will be available Sketch...

Clicking Sketch... on the image you want to edit will open the Sketch for Confluence editor.

To use any tool provided by Sketch simply select the tool from tool panel, located at the top of the Sketch dialog box.