The best I can figure out, the witches change their mind and let Damon enter the house unharmed after Stefan lets Damon in on his plans. The witches trust Stefan at this time so they are trusting who he decides to trust. As long as Damon wants what the witches want, he will not be harmed in the witch house.
Stefan: I´m in this alone, Damon.
Damon: You´re go after Klaus you have to be cutthroat and devious. I´m so much better at that than you. Come on, brother. What do you say? If you're gonna keep saving my life, at least make it for a good reason.
Stefan: You want in?
Stefan: Okay. But it´s just me and you. Elena stays out of it.
Stefan: Follow me.
Damon: Wait. I´m not so...welcome in there.
Stefan: Don´t you worry Damon. We all want the same thing.
(They enter the witch house)
Stefan: Have a look.
Aside from Damon's previous comment that the witches are "fickle", there is precedence for the witches changing their minds. As seen in the episode "The Ties that Bind":
Klaus: What took you so long? Hiding behind your witchy friends. And in squalor no less.
(He breaks down, he's in pain.)
Damon: Insulting a bunch of dead witches... not smart. I made the exact same mistake first time I came in here.
Klaus: Well, you know, the funny thing about witches is, that living or death, they care about their own. A hundred dead witches have a thousand living descendants... And I have no problem, killing every last one of them, if I don't get my coffins back. As we speak, my hybrid friend is prepared to end the Bennett line.
(The candles die down, Klaus isn't in pain anymore.)
Klaus: Now... please... show me the coffins.
(Now he can see the coffins.)