I'm trying to make IMBC and it's not going well.  I beat the egg whites until stiff, heat the sugar syrup to 240, add that to the egg whites but it never sets up after that.  I wait for it to come to room temperature by feeling the mixing bowl but when it's the right temperature to add the butter it looks like soup!!!  What am I doing wrong?   

It might not make much of a difference, but I take the sugar syrup up to 250 when I do IMBC. 

If it's really soupy then there's probably just more heat than you think in the bowl. If you're using soft butter the meringue really needs to be cooled down. When I make it I use cold butter I take it out of the fridge when I start cooking the sugar, pretty much. That way it's not going to end up soupy. It might end up looking curdled for a while as it's mixing, but it's a lot easier to fix a broken buttercream than a soupy one. Here'e a video showing how I make it, and one showing how to fix it if it breaks: