Have you requested access to the concrete5 project through this page:
http://mygengo.com/string/p/concrete5-1/access/request...

Include there the language you're about to translate together with your concrete5.org account name.

After that you should be able to access to translate that language in myGengo. This should happen through this page after logging in and having the access to the correct language in the concrete5 project:
http://mygengo.com/string/p/concrete5-1/dashboard...

Then just select the correct concrete5 version you want to translate and you should get a translation interface in front of you.