How to Enable Dictation in OS X Mountain Lion
Turn on Dictation in OS X 10.8
Click on the icon and choose system preference then select 'Dictation and speech' a box will apear, select "on"
a confirmation dialog box will appear, choose "Enable Dictation"
Start Using Dictation
The default key for dictation is "fn" but dont panic if your keyboard doesn't have the button. you can simply set any key within the Dictation & Speech settings.
Be aware that some noise in the background might interfere the speech to text so always make sure you are in a good environment to get desired results.
If you have more tips on how to use dictation, please drop them in the comments.