See > http://support.apple.com/kb/ht5449 Looking at your OS X version, I see that you are running OS X Lion (OS X 10.7.5), not OS X Mountain Lion. To confirm this, open > About this Mac, and tell me what you see next to "Version".
If you are using OS X Lion, you have to upgrade to OS X Mountain Lion to get Dictation. First, check that your Mac is compatible > http://www.apple.com/osx/specs Then, make a backup of your data and check that your applications are compatible > http://www.roaringapps.com
After doing these steps, open the App Store and purchase OS X Mountain Lion. When the installer finishes downloading, it will appear, so follow its steps to upgrade.
Another option (more expensive) is to stay with OS X Lion and buy Dragon for Mac > http://www.nuance.com/for-individuals/by-product/dragon-for-mac/index.htm