From Second Life Wiki
Most Macintosh systems have a line level microphone input that requires a powered microphone. Most headsets using 1/8" plugs do not have line level microphones. USB headsets are the simplest solution for Macintosh users. USB to 1/8" adapters are also available. Typically drivers are not necessary for the Macintosh to use USB audio devices.
To select your USB audio input and output devices look in System Preferences and select the Sound option. Both input and output may need to be selected.
The input section also allows adjustment of the "Input Volume" for your USB device. Output volume will be adjustable using standard adjustment options found in the Macintosh interface or with keyboard volume controls.