You should see a menu like this:
Now tap or click
Select a server. A drop-down menu should appear including all servers you got
Manage Serverpermissions in. Select the server you want the bot to join. Afterwards click continue.
After selecting a server, you should see something like this:
As long you are not experienced with Discord's permissions system, keep all permissions selected. Click the Authorize button and complete the Captcha.
The main reason for your server not appearing in the drop-down menu is the use of the wrong account. Make sure you're logged into the right account which you can check on the top of the invite screen. You should see your profile picture and Discord tag. If it's the wrong account, press the Not you? button.
If you like to add a bot to a server you require an account with a verified e-mail. Head over to your user settings and verify your e-mail.