All the social media icons are available in the standard image gallery area within the design studio.
Simply follow these instructions:
1 - In design studio click the IMAGES tab on the left
2 - Then click the IMPORT images button on this tab
3 - The image gallery will appear and you should be shown the social icons to import
Click the ones you want to import and then click the import button at the top.
These will all then be in your design on the left hand side image window:
You can then add these to your design.