Because of the way the runway extends to the end of the land fill, the real world KBOS localizer for Rwy 15R sits about 220 feet to the LEFT of the runway.
Checking Google Earth images from 2002 which the Nav data for FS2004 was created - the localizer appears in the same location. FS2004 got it wrong - on the wrong side of the runway. You should be lined up slightly left of the runway, but it really doesn't not matter.
This is known as an Offset Locaizer - but because it is such a minor distance - it can be used as an ILS. But real world rules do not allow this ILS to be used for autoland or CAT II or CAT III precision approaches.
The localizer will bring the pilot in close to the runway - but the final lineup with the runway must be made visually.
@ PawPaw's house - near KADS, Addison, Texas, USA