If you're using the older Explorer client (/alfresco) then tagging is a little tricky, but can still be done.
- Choose Run Action, then Apply Aspect - Choose the "Taggable" aspect and finish the wizard
On the details page you'll then see a "Tags" property. If you edit the data (clicking the icon in the top-right of the panel) you can then click the empty tags label to bring up a tag picker control.
All this is easier in the Share client (/share) which has much better native support for tagging.