how did montmorency contribute to the packing?

The packing escapade was already a messy job and Montmorency's contribution made it even more tiresome. He did his level best to be a “perfect nuisance”.

He came and sat down on things just when they were about to be packed. According to the narrator, the dog had a belief that whenever Harris or George reached out their hand for anything, it was his cold damp nose they wanted.

Montmorency put his leg into the jam and disturbed the teaspoons.  He pretended that the lemons were rats and went into the hamper to kill three of them before being hit with a frying pan by Harris.