You should be able to send a text in the desktop app or website under the messaging tab, by clicking the plus button and "New SMS":



On mobile, you should see a "New SMS" button when you go to send a message:



If you don't see these buttons, try quitting out/swiping away the app and restarting it, or restarting the device, and see if they show up.  Sometimes the app doesn't get the memo that you're licensed to send texts until after it restarts.