After purchase, download the .zip file from your Elegant Themes account.
You will install this plugin just like any other WordPress plugin. From your WordPress Dashboard, go to Appearance > Plugins> Add New and click on Upload Plugin. Locate the .zip file on your computer and click Install Now.
Once the file upload is finished, you can click on Activate. You will then see Divi Menu Plus as an active plugin in your Installed Plugins list.