How to create a card from the Library?
1) Click on the arrow just beside the word +CREATE
2) Choose Content
or You also can directly click on +CREATE 
3) Select the Card you want to create in the library.
Or click on Upload Media in case you want to upload content to your library.
Digital Signage - Appspace Libraries:
How to Include Google Sheets card :
- Specific process to be followed
- Requires to share the spreadsheet with sharing account
How to Include Google Presentation card :
- Specific process to be followed
- Requires to share the presentation with sharing account
NOTE : Slide transition effects work, but animation in Slides do not work in Appspace
