I want to make my object's collision, and I have one problem: My XSI got messed up and, well, you know how it generally picks up every object touched by the rectangle in rectangle select, visible and covered? Well, that only happens to the front now. But I can't select everything at once, and when I try to click the "All" button in the top left corner, everything is selcted, inclusing the dummyroot, so I can't merge it. Help?

try to select them via the explorer (8)

Okay, but how do you select more than one in the explorer?

same way as you do in windows: press STRG and leftclick the objects