can i download powerpoint to use on macbook air

Microsoft sells a version of Office for the Mac. You can purchase it either standalone or as part of an Office 365 subscription. They also offer a free, feature limited web based version at office.com.

Or you could try an alternative like Keynote, which is free.

Note: If you are actually running macOS 10.6.8, you will not be able to run the current version of MS Office. It requires macOS 10.10 or higher. Older versions that would run on your machine are no longer supported by Microsoft.