Using the mybook to copy files from a mac to a pc

brought a mybook essential 1TB today on the salepersons word that i would be able to copy files from a imac onto the drive and then copy onto a pc. have i been taken for a ride from the salesperson, or is this possible? it seems that imac doesnt allow me to paste from the imac hardrive onto the mybook drive. and i cant access the files through the windows operating system on the mac to copy onto the mybook either.

help please?!?!?