Alright:
When you download the mod, you get a .zip containing 2 folders. 1 folder is the standard version and the other includes OPCF. 1 of these 2 needs to go into your JSGME "MODS" folder. You can place them both there, but only enable 1.
So in the left pane of JSGME you should get "MRP 1.2" and/or "MRP 1.2+OPCF". Select 1 and enable.
I suspect you extracted the .zip into a new folder and placed that in JSGME. Most likely it would be called "More Realistic Periscope 1.2". That would cause the folder structure to become messed up, and the files end up in the wrong place.
To put it in another way: the folder containing the "data" folder needs to be moved into JSGME "MODS" folder. For example:
C:\downloads\more realistic periscope 1.2\
MRP 1.2\data\...
Hope it helps.