When you get started with interior building, it certainly isn't fun to track down all of MarbleBlast's textures and import them, especially since Constructor will crash if there are any .dif's in the folder you are importing from. I also had trouble finding a link to marbleblast_map2dif here on the forums, and had to ask for it over MSN. Here are some links to some textures and map2dif, to make setting up a little easier for people who are just getting started.
Here are all the textures found in the marble/data/interiors folders of MarbleBlast Gold, Platinum and Advanced.
To use directly from Constructor (in Windows), rename marbleblast_map2dif.exe to map2dif_plus.exe and drop it into Constructor's root folder, overwriting the map2dif_plus that is already there (or rename the original if you'd like to keep it).
To use on a Mac, PerishingFlames has a nice guide here: