I have Audacity 2.0.3 running on a Macbook Pro under OS X 10.6.8. I also have a large quantity of commercial music cassettes with Dolby Noise Reduction, and a very inexpensive USB cassette digitiser that does not de-code the Dolby. Is there any way I can compensate for the Dolby Noise Reduction in software within Audacity (or otherwise)? Would be very grateful to know

“Dolby” encoding/decoding is restricted by patent/copyright, so any official Dolby encoder/decoder would need to be licensed from Dolby (and would probably not be free).
Just reducing the very high frequencies a little using the Equalization effect will usually be sufficient.