Open Sidekick. On the top click on Setup, Preferences. In the pop up box that appears click on Email to the left. If you have Gmail mark that option at the top. If you have another email choose SMPT
If you have an email other than Gmail you will have to find out your outgoing SMTP, Port, User name, email, and password. You can find out the outgoing SMTP and port normally by searching support or calling support for your email provider. Yahoo does have this information available online and when logged into your yahoo account.
If you have Gmail you need to turn off 2 step verification and allow less secure apps. We do have another FAQ with how to do this.