What are you trying to get the entity to do? - and where did you get the model for the x.file from?
If the model is invisible inside FPSC there are useually only a couple of reasons for that.
In the case of flying models I use - some use a path - some dont but fly independantly. None of them are currently set to be enemies though the flying entity in the screen shot gallery hurts the player on contact.
If you can load your gib entity as a static entity and see it then theres no reason it should not appear if you turn it to dynamic.
Remember though that FPSC does not like you messing around trying to get flying objects to appear and behave as you want them as it dont really support such things by default. If you can see the entity as a static one then when you turn it to dynamic and you cant see it - its probably dissapeared somewhere out of view because the physics or other settings are not correct - entities not set up correctly can disapear because they fly out of the players view or even out of the level.
All of the objects I have tried to get to fly have behaved in this way - some tips that may help everyone.......
Collision On or Off also plays a big part in entities not staying where you put them so adjustments to numerous settings have to be made or they will dissapear.
If you want your entity to fly - think about it? How will you achieve that.........
You could start by trying to change the default appear1.fpi script to
"appearnogravity.fpi" writing your own script if necessary.