OK, great! I figured out the problem and will share it here in case it happens to anyone else... which is unlikely... but possible:
I have the CS5.5 Adobe Suite. Some time ago, I installed a trial version of Premiere (CS6), which also (it seems) installed the CS6 version of Extension Manager, which muscled in as the default program for installing Adobe Extensions. Therefore, when I downloaded the Materials package, that's what opened and tried to install the extension. It failed, and told me I needed at least Photoshop version 12... which was confusing as I have that... but the CS6 Extension Manager didn't 'see' any of the CS5.5. products.
So I just located the 5.5 version of the Extension Manager manually and installed the Materials package through that.