The ZnSe lens sits in front of the Seeks Standard lens (not instead of it!) and you set the manual focus near infinity. Move towards the target until the image is either in focus or close to focus. Adjust manual focus on Seek until good focus is achieved.
Note that a 50.8mm Focus point lens will not provide perfect focus at 50.8mm when used as a supplementary lens but it will be close to that distance.
Fraser