To search your messages, simply tap the magnifying glass in the upper right-hand corner and begin typing in the search bar and your results will live filter.
If you have more than one Tresta number, you can also filter on only messages associated with a subset of your numbers by tapping the more icon in the search bar and selecting only the phone numbers you want to include. When you are filtering on a subset of your available Tresta numbers, a blue dot will appear on the more icon to let you know that you have an active filter applied.