It seems the Sibelius presets for the DImension Brass are incomplete. The matrices for the presets associated with keyswitches D#1, F1, F#1, and G1 are empty. Also the keyswitch has no effect on switching to the correct matrix when invoking the .2, .3, .4, and 3.+4. dictionary items in Sibelius.
The above comment is true for all the Sibelius presets in the Dimension Brass (e.g., all the presets that begin with .Sib_DimBr).