I would first see what driver is loaded
# airmon-ng - you should see rtl8187
make sure your usb is seeing your Alfa
Try a modprobe for your diver and remove any other driver that might be loaded.
# rmmod xxxxx && modprobe rtl8187
unplug the usb to the Alfa and plug it back in.
try #airmon-ng - you should see rtl8187
put your Alfa into mon mode.
#airmon-ng start wlan0
# airodump-ng mon0
To get back into managed mode:
#airmon-ng stop mon0
#airmon-ng stop wlan0
# /etc/init.d/wicd start
go into the menu and press the wicd icon find your essid and a press on it. Enter your pw.
#iwconfig and you should see your card in managed mode with pw etc
This is all really overkill but it should get you on your feet.