Grooved side of the fresnel should face the ground glass.
A fresnel eliminates hot spots and opens up the shadows making the overall scene lower contrast than it really is.
The rings of the fresnel can make seeing the fine detail of a scene harder to see when using a loupe on the ground glass depending on how fine the rings are on the fresnel and how well you can "see".
I had a Meridian 45B. I bought a reading fresnel from Office Max. I cut the fresnel down to fit the ground glass opening of the camera. Upon removing the ground glass I discovered the fresnel would fit between the frame and bosses for the ground glass without moving the ground glass position. After modifying the edges and assembling I checked the focus with a loupe from infinity to a few feet and no shift in focus.