Enable the clock indicator:
gsettings set com.canonical.indicator.datetime show-clock true
Set the indicator to use the custom format:
gsettings set com.canonical.indicator.datetime time-format custom
Set the actual format to use:
gsettings set com.canonical.indicator.datetime custom-time-format "%A, %B %e %l:%M %P"
NOTE: The different options that can be used in the format string can be found on the strftime function man page.