It really doesn't matter, but if you have a Solomon Islands scenery layer containing Maskrider's Solomons scenery that would be a logical place for it. If not, you could put it in the SCENEDB\afdfiles\scenery folder.
One thing to remember is there is a limit to the number of AFD files you can have active - approximately 200. Consequently I keep all my AFD files separate from the airfield scenery files to which they refer. I have all my add-on scenery infrastructure files active but only one or two AFD layers, which are grouped in geographical areas, active.
Here is part of my scenery.cfg file showing the Solomons and Australian scenery active but only the Solomons AFD active and the Australian AFD inactive:
[Area.056]
Title=Australia AFD
Local=ADDON\Australia AFD
Remote=
Active=FALSE
Required=FALSE
Layer=56
[Area.057]
Title=Australia
Local=ADDON\Australia
Remote=
Active=TRUE
Required=FALSE
Layer=57
[Area.058]
Title=Solomons AFD
Local=ADDON\Solomons AFD
Remote=
Active=TRUE
Required=FALSE
Layer=58
[Area.059]
Title=Solomons
Local=ADDON\Solomons
Remote=
Active=TRUE
Required=FALSE
Layer=59
Flatten.0=15.9,S9 6.88,E159 14.24,S9 6.20,E159 14.21,S9 6.16,E159 14.86,S9 6.84,E159 14.90