I have the CD 32 version of Wing Commander and although there is an option of playing the secret missions, in fact you can't. It must be a leftover from the PC version.
For everything WC related visit:
And if you want to play WC on you PC, Home of the Underdogs
hosts a cut down version of Kilrathi Saga. (Or even try e-bay. You can find the original Dos version at reasonable prices.)
And of course do not forget the freeware remakes of the original Privateer, Privateer Remake http://wcuniverse.sourceforge.net/
and Privateer Remake - Gemini Gold
(Images provided from Wing Commander CIC)