Normally the "cleansed" version of the file system that the Keychain's Get Info dialog presents to you won't allow you to access the hidden /usr/bin directory, but I found a way around this.
- Get normal Finder windows to show all files. If you aren't aware of how to do this, check out this article.
- In a normal Finder window, navigate to /usr/bin
- Drag bin over to the Places area in the sidebar. Now bin is a shortcut you can access from anywhere.
- From within the Keychain's Get Info -> Access Control pane, click the "+" button to open the find application dialog.
- Click the bin that is now under the Places on this sidebar.
- Navigate to and select codesign.
- Click the Save Changes button.