Quote: "how to load models for FPS creator in DarkGDK?"
dbLoadObject.
Seriously, it's that simple. I don't use FPSC but I have a few FPSC model packs which load fine into a Dark GDK program once you figure out what's in which directory.
The texture files should be in a "texturebank" directory, mainly in DDS format. Also look out for "fpe" files (don't know which directory, maybe the same as the model). These are text files which tell you the name of the texture file which should be used for the model, and also the start and end frames of the animation sequences of the model.
The X files are usually human-readable text files as well, and they may contain also a texture file name. If you search for the second occurrence of "texturefilename" within the X file, that is the texture that the model will attempt to auto-load. If the texture is in the same directory as the model, then with dbLoadObject you get both model and texture. If it doesn't succeed for some reason, you can still load the texture as an image and then use dbTextureObject.
It's worth to cross-check the texture file name in the X object and in the "fpe" file, because I have found in the model packs a few models which had wrong texture names in the X file but the "fpe" was correct. Since the X file is also text format, you can even correct it, to make the texture auto-load.