apt-get install hostapd
Then proceed setting up hostapd from command line. Can't be done from Zentyal GUI. Google helps with the configuration of hostapd.
Also, wireless card must be itself hostap-capable, you can't just use random card and expect it to work as access point. Most cards do not. Atheros cards work best, some Ralink cards work as well but are somewhat buggy. Personally using dual-band Atheros AR9462 over Mikrotik RB14EU host-adapter card.
Instead of WEP use WPA+WPA2.