If your model has only a single texture of any kind you can pack the texture into the FBX and it will work without issue 99% of the time. Set Path Mode to Copy and push the button right next to it that enables packed textures.
If your model has multiple textures, packed textures will not work (Blender produces a screwed up FBX when packing multiple textures for some reason, not HiFi’s fault). EDIT: Apparently this is a bug that doesn’t happen to everyone, others below have had no trouble packing multiple textures into the FBX, try it and see if it does for you.
If you don’t want to pack textures into the FBX, Set Path Mode to Auto. You will need to create a FST file that tells HiFi where the texture directory is online (a Photobucket album will usually work). Look up the FST format on the Wiki or here on the forum for more details.
Also, forgot to mention, if your model does not have anything transparent on it, make sure Transparency is completely off. If it’s turned on when it shouldn’t be (and you do not have an alpha texture set), your whole model will be invisible. If your model does use a non-opaque material, use Mask transparency mode, not Z-Transparency or Raytrace.