Are you sure the network is available? Did you try to reboot? Sometimes the network gets stuck in non working state.

Starting a particular connection is only possible for already setup connections. It works by sending commands to the connection manager, so it will probably not work if the connection manager does not work.

The command is:

dbus-send --type=method_call --system --dest=com.nokia.icd /com/nokia/icd com.nokia.icd.connect string:$1 uint32:0

where $1 is replaced by the id of the connection (find it by running the command:

gconftool-2 -R /system/osso/connectivity/IAP | less