Ubuntu 8.10 will automatically detect your sierra aircard wireless modem when you plugged it to your PC, but there are some steps you should add to connect to internet :
1.plug in your modem to your PC
2.open terminal, type : sudo wvdial.conf, this will automatically create a file /etc/wvdial.conf
3.edit the file using a text editor, i used gedit (coz i was a windows user, and this text editor is the most convenient for me ^_^ )
4.type sudo gedit /etc/wvdial.conf, and type this configuration :

i dont know the whole meaning of those configurations, i just found it by google’s search ^_^, anyone knew, pls tell me
5.Click save

happy surfing… :))