When you are in the "Call Log" view, you will see a row of icons at the bottom of the screen. Next to the phone icon on the far left is an icon that looks like a cell phone. If you tap on that icon, it will open up a menu. Click the "Text Log" link in that menu to get to the text message log.
  
Once the text log loads, you will see a similar view to the call log with text messages. You can either send a new text message or open up a current conversation and respond.