Most conversions are really straight-forward. Just start Quartus and open the MAX+PLUS II project. When Quartus sees the MAX+PLUS II .ACF file, it will automatically convert the project to a Quartus project. If it has problems with the conversion, it has fairly detailed messages to explain the problem.
Note that not all the old devices that MAX+PLUS II supported are supported in Quartus. The transition from MP2 to Quartus happened around the time of the FLEX 10K parts and the MAX 9000 parts, so if you're project is from that era, you may not have support.