I have made shield for our caps in the Wing Commander mod and they work fine. I can see them in modelview, I have got the shield entry in the table file and remember to activate it in FRED2 !!!
Since Caps normaly have no shields in FS2 they aren't active when you make a mission. Klick the ship in FRED2 and to edit ship, there you will find a shield "on/off" button.
I have also made this mistake first and wondert why the shield isn't there. Second thing is, make the shield hullshaped and quite close. Every fighter pilot that whants to get trough that close shield will colide with the hull and bomber will blow away themself with there bombs.
Beside that, is it possible to make weapons only shield penetrable or the opposite.
Example: Energie weapons from fighters haven't got a chance against shields.
Only Torpedos can get trough the shield and damage the ship.