How to log in to a Mac OSX desktop with no user account password?
Resetting User Account Password (10.7 +)

1) Boot into your Recovery Partition holding down command + R at the startup chime

2) Select 'Terminal' from your Utilities Menu Bar

3) Type the following command in: 
     resetpassword

4) From the window that appears, select your boot volume and the appropriate user account

5) Type in the new password and confirm the new password

6) Click Save, close this pane and quit Utilities

7) Select your boot volume when presented with choose startup disk and reboot your machine