Once you have some subscribers added to your account you can start sending messages.
To send a message, go to the Messages tab and click the "send" ("paper airplane") icon toward the bottom right corner of your screen:
Next, select when you would like the message to be sent:
Select whom you'd like your messages to be sent to (individual contacts, segmented groups, or all subscribers):
Select whether you'd like your message to be sent as SMS ("regular" text) or MMS (picture message, which allows for more characters and counts as 3 message credits each).
Then type out your message content and tap "Send."