Adding a Library in the Mac app
Open the Preferences window by choosing Sketch > Preferences (or pressing ⌘+,) and click on the Libraries tab. From there, you can either drag the Sketch document you want to use as a Library into the window, or use the Add Library… button to browse for it.
To find out more about the options in the Libraries tab, take a look at the Preferences section.
Adding and Installing a Library from the web app
To add a third-party Library from the web app in the Mac app, head to the bottom-right side of the document’s sidebar and click Add Library to Sketch.
For Libraries in your Workspace, clicking on Add Library to Sketch will install that Library directly in the Mac app.
Libraries in My Drafts folder
When you create a Library in My Drafts, it will be available as a Library just for you. This is handy for those elements you use frequently in your designs but that are not necessarily interesting or helpful for the rest of the team. This way, you don’t add unnecessary files to your team’s Libraries list.