I'm trying to model an unbaffled stirred tank reactor, into which I want to inject a stream of particles. However, although I create a new material in the Materials dialogue box, that material is not available to me when I try to create an new injection. What am I doing wrong (this is driving me mad!!!)

You need to create a material with the material type set to "particle" (either inert particle, or droplet particle). These option is only available in the material panel after you have created an injection, so you will need to create your injection, then create your material, then go back to the injection and change the material.

Rob