Hi, I'm looking for some more help with my first Portal 2 map.

I need to know how to set up a trigger that can only be set-off by a cube.

Thanks.

Create a filter_Class and give it a name.
For the filtered class use the classname of the cubes (prop_weighted_cube I think).

In the triggers keyvalues you have to look for "Filters" and there you chose your cube filter you set up above.