I am using Sibelius 6 and Kontakt 2.2.4, but when I load the Cymbal Rack Combo into a score, all I get is suspended cymbal sounds. When I open up the Sibelius Mixer, it lists the Cymbal Rack Combo as 'Marimba Percussion'. I'm not sure how to fix this, or what is causing it.

are you using a template from thwritescore.com and the full version of VDL (not the included VDL sounds in Sibelius) ?

Yes, I am using the 2.5.2 updated version and the Sibelius 7.0 Templates from thewritescore.com. I exported the housestyle from the templte into a score I was already working on, but all the other instruments have the correct mapping and sounds.

Here's the workflow that works for me: create a new non-template score, then import the house style, then press I to add an instrument, then make sure the ";choose from:"; has VDL Template selected when choosing the cymbal rack.  Then I would copy+paste from the working template file into the new one.  

However, the official README file (appendix D) says it's better to add instruments to the existing template file, rather than messing with house styles.  For example, I would add band parts to my drumline score that currently works, before trying to add drumline parts to a band score that was not made with the template.