Authentication requires a valid OAuth application to be registered with http://www.assembla.com.
Using your Assembla account you can create a new application registration using this link: https://www.assembla.com/user/edit/manage_clients
Registered oAuth application ID/Secret will be visible only for user who created it.
The entry should look like this:

Name	You can set any name you want.
Website	http://127.0.0.1:8080
Redirect URI	http://127.0.0.1:8080/securityRealm/finishLogin

With 127.0.0.1:8080 replaced with the public hostname and port of your Jenkins instance.

The /securityRealm/finishLogin is required as this is where Assembla will redirect the user for the second part of the authorization process.