Old versions
The current version of Pear Note for Mac requires at least Mac OS X Mountain Lion (10.8) to run. If you're still using Lion (10.7) or Snow Leopard (10.6), you can still use Pear Note for Mac but you'll have to use version 3.1.4.
If you're still using Leopard (10.5), you can still use Pear Note for Mac but you'll have to use version 2.