In order to allow your vBulletin members to switch between the installed forum front-end styles navigate to your vBulletin admin panel->Settings->Options->Style & Language Settings->Edit Settings.

Select the Yes radio button in the Allow Users To Change Styles area and save the modification. In this way when you log in your forum's front-end you will see the drop down menu in the bottom left corner and you will be able to change the styles through it.