Last week, Security Onion repository moved from Google Code to Github. We have now updated our Security Onion Installation Guide with the new links.
Basically, replacing the base part of each link from the old reference
"https://code.google.com/p/rest_of_the_link]"
to
"https://github.com/Security-Onion-Solutions/rest_of_the_link]"

If you are looking for "issues", not only do you need to replace the base reference as mentioned above, but you also need to remove the "detail?id=" at the end of the URL.
For example:
https://code.google.com/p/security-onion/issues/detail?id=488 (does not work )
Becomes
https://github.com/Security-Onion-Solutions/security-onion/issues/488 (works)