I hate to tell you this susie, but you bought the wrong one. the $30 encoder is not for MC5, if you scroll down to the bottom of that page, it specifically states that the one at the bottom is for MC3-5 and it's not a download of the encoder itself, it's just an activation. all you have to do is activate the one already installed in MC5.
you'll have to email or call cakewalk customer service to get this fixed since you only need the $19 activation instead of the $30 download.