This is where Sketch stores Plugins, Templates and Textures
To import a bunch of textures (say the whole Sublime Patters pack found at
- http://subtlepatterns.com/downloads/SubtlePatterns_volume_3.zip
- http://subtlepatterns.com/downloads/SubtlePatterns_volume_2.zip
- http://subtlepatterns.com/downloads/SubtlePatterns_volume_1.zip
Extract them all into
~/Library/Application Support/Sketch/Textures