He has claimed that he wanted to disrupt/sabotage the future recruitment of new labour party members. That's why the youth camp of the labour party was his target (labour is currently in government). In general he is blaming the party for being too liberal about the immigration policy. Quoted directly from Breivik:

http://edition.cnn.com/...ailure.cnn?iref=allsearch

From my point of view he might have thought that targeting ethnic Norwegians would have a bigger impact on the general population than targeting minorities (i.e. attacking a mosque or synagogue). Not sure if his plans will succeed though. Then again there may always be some lunatic or another who will follow his example.