When I go to the “Manage Application” > "Email" tab, there is no email address for me to choose from in the “Email Sender Address” field. How do I fix it?

Please be sure the "Organization-Wide Email Addresses" is set up in Salesforce.

Login to Salesfoce, go to "Set Up"  menu at the top. Then go to "Administration Setup" > "Email Administration" > "Organization-Wide Addresses" on the left side bar to add an organization wide email address.

Enter "Simpplr" in the Display Name field.
Enter a valid email address.
Make sure "Allow All Profiles to Use this From Address" is selected.

Salesforce will send a verification link to the email address. Please make sure to verify your email address for Simpplr's email notification to work.

Once an email address is added, you will see the new email in "Manage Application" > "Setup" > "Email". Select the new email address and click "Save". Simpplr email will use this "From" email address.