Follow the below mentioned steps to add a new slide to the project :
Step 1 : Go to Appy Pie Design
Step 2 : If you have an account login to your account, else create an account
Step 3 : Create or open an existing design/project
Step 4 : Select a category from the category drop-down menu present on left panel
Step 5 : Click on the + icon present on the right bottom of the editor
Step 6 : You will see a new slide, now you can select your design
Step 7 : You will see a slide with design. Once you are satisfied click on save icon