If this is a new install, you will be greeted with the username/password screen.
If this is an existing install, click the 3 Bars (upper left) > Settings > Accounts > Add
Skip the proxy settings for this configuration.
Select the "SIP UDP" radio button from these options.
Click on "Finish" to complete account setup.
Once your account is set up, you can start making and receiving calls from the Zoiper mobile app.