How to add a task to a checklist

To add a task to a checklist:

Go into the checklist
click on the orange '+' button at the bottom right of the screen:

The Add task screen will open.
Start typing the name of the task you wish to add. At any point you can select any item from the auto-complete drop down:

Click on the orange + button to add the task. A confirmation popup that the task has been added is shown in the middle of the screen.
If you want to add multiple tasks, simply repeat the above steps until done
Click on the back arrow (top left) to return to the checklist page and view your added tasks.

Tip: Did you know that you are not limited to adding tasks to checklists? You can also add tasks to other tasks thus creating sub-tasks? To do so, click on the task you wish to add sub tasks to and on its page, click on the + button at the bottom right of the screen in a similar way to adding to a checklist. This will create sub-tasks. Try it out.