I do not see the library (.a) in the cornerstone, so I can not add it. Ho, can I get Cornerstone to see him?
You should display the ignored items by clicking the small button on the bottom toolbar:
This is because ".a" is on your global ignore list. You can edit the list by going to Settings-> Subversion-> General-> Global ignores.
When you delete ".a", here it will no longer be ignored.