The "blogger" who posted that Mr. Scalise talked to a white nationalist group got the "story" from the son of the Democrat who ran against Scalise in Louisiana. They had this "info" during the campaign but didn't run it because they knew it to be a lie. Their supposed source, whom they won't name, supposedly had a photo of Scalise at the event, but oops - it got accidentally deleted! So, conveniently, there is no evidence of their libelous claim.
The blogger posted this lie in order to paint the new Republican majority as racist because Scalise he was going to be the House Majority Whip. Scalise wasn't there to talk to a white supremacist group, but spoke earlier in the day to a neighborhood association. The person who organized the EURO conference just also happened to be president of the neighborhood association and used the conference availability to set up a meeting of his neighborhood's association.
In short, this is a left-wing hit piece backed by no evidence.