Welcome aboard!!!!
I'll try to help but I'm an old skool modder and there are better newer ways of alt4erign FS2 so while my info
will work, it's woefully out of date.

First, is your mesh textured?? If not pls go back and cover it with a temp texture (even paint object works just so your mesh is assigned a texture slot you fill with basically anything).
Second your mesh will be difficult to manipulate in Fred and in game unless you make a copy of the texture as a 8 bit PCX file. Otherwise your ship will be invisible (same as if you typed "invisible" in a texture slot to make certain parts not seen)
Third, you need to alter an existing table file or add your own custom one. (unless you already added or imported a shield mesh, make your shield entry 0 (zero) otherwise for testing ships will fire beams through your ship, this is a bug I discovered as the game has to damage a shield first that isn't really there

)
Fourth, have fun! If you get stuck after this let us know! This should be a helpful link:
http://homepage.ntlworld.com/karajorma/FAQ/intro.htmll8tr!
"FIRE! BOMBER!!!"