If you use IVR (Interactive voice response) on your Asterisk / Trixbox setup, Enable Direct Dial option allows callers to directly dial extensions.
By default it doesn't allow to dial-in into conferences. If you try you will get response similar to:
"That option is invalid. Please try again"
This can be changed by adding following to your /etc/asterisk/extensions_custom.conf file:
include => ext-meetme
ivr-2 can be different in your case. You can find what it is in /etc/asterisk/extensions_additional.conf