Your missions have to be in the Wing Commander Saga\data\missions directory. Then you can access them in the Tech Room by clicking on "mission simulator", then on "single missions".
My Laptop:
ATI Radeon HD 4570
Intel Core 2 Duo T6600 2.2 GHZ
4 GB RAM
Windows 7 Professional 32-bit
My big PC:
ATI Radeon 9800 Pro
Intel Celeron 2.0 GHZ
about 700 MB RAM
Windows XP 32-bit
Soundblaster Audigy