Initially Japan declined most of the help, because, they said, they can't guarantee the safety of outside helpers nor can they assist them in getting to the disaster zones. Nevertheless, some expert teams from a number of countries are there already, helping them. From what I've learned, the US army is helping quite a lot, having granted even a carrier to be used as a helicopter base near the affected areas. Needless to say, those helicopters and the carrier itself have already been irradiated as well...
Hard to say about the nuclear plant fiasco, though. They have receiver plenty of advice but since nobody can anymore get anywhere near the worst reactors due to radiation, it's not like it makes any difference if they get the advice from on site or via video streams... At least they think they are getting the electricity back, finally. According to our Radiation and Nuclear Safety Authority the failure to restore power for so long was unforgivable.