UPDATE: claude changed where the 'add plugins' part is slightly.
You can either:
1. Just start a new task with cowork and then drag the zip file into the chat and say 'upload this plugin'
OR
2. You can install one of Anthropics plugins, then it will give you the option to add your own plugins.
Hope this helps! (Sorry they change this stuff every day, makes it a nightmare to keep up with😅)