ACID works in the digital audio domain; that is, the loops that ACID uses are WAV files, and ACID operates on them with digital signal processing, changing key with a Pitch Shift algorithm and changing tempo with a Time Compress/Expand. ACID cannot separate the musical components of these files and alter them, i.e. change a Major 3rd and 7th to Minor without changing all of the other notes. Basically, ACID takes the chords and notes you give it, and can only transpose them up or down in their entirety. It cannot change the relationship between notes within a chord.
If you need a loop in a particular key, and are unable to find one on the ACID Loops CD(s), you can record your own loops in ACID, and edit them either in ACID or in Sound Forge.