Others may be able to add more but, after reading your Spiratone link, it appears that the fisheye screws into the filter adapter. You have to have the standard or after-market 49mm filter holder adapter already on your X100 and yours probably has it if you use any filters. You just need the step-up or step-down adapter to go from your lens to the fisheye.
It looks like the fisheye was sold with adapters of different size. You may find the size e.g. 52mm on the adapter or you can measure it with a mm ruler. If it happened to be 52mm, you would need a 52mm to 49mm step-down adapter, available at photo dealers, eBay and other places.