To add a Text Content Block to your slide, select the "Add Text" option from the "Add Content" dropdown button. A dialog box will pop up appear that lets you create a new text block.
You can enter the text you want to add to the block in the text field of the dialog box that appears. Once you are done, click on the "Submit" button to save the Text. Once you submit the text, you will be redirected to the same Slide page in which these Content blocks will be listed one after the other.