While in the References tab, click on the selection box above the search bar to access your collections and groups from your Mendeley library.
The All References collection is usually selected by default when you launch Mendeley Cite in your Word file.
You can select a Collection or Group in the same way you would in your Mendeley Library.
Make sure to click on the arrow at the top left to go back to the References list.
Every time you select a Collection or Group, its name will show inside the selection box above the search bar in your References tab.
All references in the Collection or Group folder selected will be listed below the search bar, and can be cited in your word document.