What are they doing? A lot.
I do not know how many e-mail accounts he had, but the police is of course working on it. There are about 100 detectives working on the case, and then we have all the forensic technicians (if that is the correct English term) and support personnel. The police have according to Norwegian law declared everybody at the island, about 700 people (mostly youths), victims of the crime. Also declared to be victims are the close relatives of those that were there, everybody at the scene of the bombing, everybody that were supposed to be there, everybody that usually works there, the rescue personnel and volunteers involved at the island and some on the bombing scene, and probably a few I have forgot.
Since they are treating each kill as a separate murder and are going to collect evidence on each of them, all of these people are to be interviewed by the police. In addition they have spent two weeks searching the entire island square inch for square inch for shell casings, other evidence and all electronic equipment that might have captured parts of the event. I suspect that they are going trough all the phone logs at and around the island as well.
Then they have set out to map all his movements and actions over the last nine years.
So, that is what they are doing. If you really wondered. About his e-mail accounts, Google contacted them almost immediately to start the work of releasing all e-mail and information associated with it. As far as I know this process is ongoing, if not resolved already, between the police, the Norwegian government, the US government and Google. Even in an open and shut case like this, an American company can't just hand over information to a foreign power, even if the information is about a citizen of said foreign power.
The last thing we need is for these other criminals to come and "help" the police. It is a waste of time and resources. There is a reason this is done by the book, and all this does is creating a web of dead ends. All accesses to these accounts has to be investigated since there is a chance they could have been done by accomplices.
In addition these hacks are of course a crime committed against Anders Behring Breivik and I think that is the last thing the police wants to investigate right now.
The people that did this are muppets, bleedin' muppets. And if I ever...what? Yes, that is my hat and coat...