You have to know the BSSID.
The ESSID is for example something like "my_home_wlan".
The BSSID is the MAC-Address of the Access-Point and so the BSSID is ALWAYS shown.
Watch this picture:
On the right-side you have the ESSID. The Owner of the Access-Point can enable "hide ESSID", so that you have a "<NULL>" or just nothing printed on the right side.
But the BSSID is, as I said, always shown and most of the Backtrack-Software uses the BSSID.
And the reason for this is simple:
Most home-user don't change their ESSID and leave it the default-value.
Let's say, that your neigbours are using the Linksys W54G-Router.
So you then have 3-4 WLAN-Networks and all have the default "linksys_w54g"-ESSID. If you tell the your Computer "connect to linksys_w54g" the Software asks itself "yeah... but... which one of these 3?".
The BSSID (MAC) is UNIQUE, so it's definite.
I din't used "wicd" before, but it should have an option, where you can put the BSSID in. If you can't find the right textbox to enter it, just try the textbox you would use for the ESSID
Hope this helps you