Hold your right mouse button over the object and select "assign new material", choose a blinn or lambert from the following menu, open up the attributes editor and click on the little black and white checkered box to the righthand side of "color", select "file" from the next menu, then back in the attributes editor next to image name selct the folder and browse to your image of the carton, should be okay from there.

If there's any other problems hopefully someone else can help as I'm off out for the evening, good luck!

*edit*

Oops, looks like Ozgalis got in there at the same time as me...lol. Either way will work.