Click on the "+" icon
Choose "Text Block" from the Content types
Type something for your students to read!
If you've created your Formative account just recently, you may see a different version on your screen, and to add a text block you will click the square icon on the left to add content, and choose from the column with the red icons :
You can also add content between other things on your formative. Simply click on the + icon (or the appropriate add icon) in between questions or content you've already created and select from the menu that appears
Tip: You can add multiple questions to the text block, so your questions are grouped together