On the upper right corner, next to the hamburger menu, there is an add (+) icon.
Click on the add (+) icon. This will present you with the following options to add from:
Project Files - Clicking on this option will allow you to add and view files that have been previously uploaded in the project.
Media Gallery - Clicking on this option will allow you to select photos from you phone. You can select up to 8 photos at a time.
Audio - Clicking on this option will allow you to select audio files from your phone.
Document - Clicking on this option will allow you to add documents from your phone's files.
Design - Clicking this option will allow you to create new deigns or use templates to enhance the look of your project.
Text - Clicking on this option will allow you to add text.
Link - Clicking on this option will allow you to enter a web address and add it to the project. You can also add a link preview cover and select a layout.
Project - Clicking on this option will allow you to link another project within the current one.