Ok! I followed this guide: http://talk.maemo.org/showthread.php?t=68002 to enable monitor mode on N900, and Aircrack doesn't work if I don't enable those drivers. As reguards fAircrack, I thought I had to enable them manually, too. However, I click on enable injection and then enable monitor mode.
ifconfig wlan0 down iwconfig wlan0 mode monitor ifconfig wlan0 up