Use one of following commands to insert a new (blank) slide
On the formatting toolbar, Click the new slide button,
Go to Insert -- New Slide or Press Ctrl + M
Note : To insert slides from other existing presentation.
Select Insert ] Slide from files
Click the Find presentation tab, and then click Browse.
Navigate to the location of the presentation that you want to insert slides and Click the presentation and then click open.
Click the slide that you want to insert, click insert, and then click close.