Hi,
It should be the same as in FSX - just add the folders to the scenery library and make sure that the folders are arranged in the right order (folder 1_xxxxxxx on top). You also need to make sure that the location of the scenery folders is outside the UAC, but that's a common problem and not specific to the Solomons.
Beware though that since P3D V2.4 or 2.5 some of the AFCAD style airfields in Solomon 1943 are missing the taxiways. LM changed some AFCAD definitions back then.
I've got it running in P3D V3.
You also can use Freemesh FSX Global and dump or deactivate the mesh folder that comes with the Solomon 1943 distribution, by the way.
Cheers,
Mark