However, selecting the material from the drop-down menu in Scene Manager, we're unable to bring up any additional material choices.
I am not clear on what this means. When you select the material, do you see something similar to this?
Note that for a brand new material (meaning, one that the plugin has not seen before) the material preview image will be a generic image which reads "NO PREVIEW". You can generate a material preview by clicking the little Refresh buttons at the lower-right corner of the preview image. From here, there are two ways to go: build an Embedded material, or link to an MXM file (i.e. a Maxwell Material file) on your disk. To build an Embedded material, you would use the buttons in this area:
Here, you can set the material's overall "Character", meaning, glass, plastic, etc. Using the other four buttons, you can change the material's color and textures. If, on the other hand, you want to link to an MXM file, click the MXM button at the window's top right:
This will show the material's MXM Mode interface, where you can browse your disk to find an MXM file to link:
Please let me know if this helps, and if not, let me know more about the situation. The images above are taken from the online help, which you can find
here.