I just got my Sprint Wireless Aircard 595U working on Kubuntu 8.04. The steps will be the same as on your distro. Kubuntu already had all of the required software. Let's say that your distro also already has all of the required software.
Log on as root. You can do this from within a normal user session. Just open a terminal window and use the su command to log on as root within that terminal.
Run the dmesg command to see the last entries in the system message buffer.
Plug your modem into a USB port and wait about 10 seconds.
Run the dmesg command again to see what Linux did when it saw the card. Hopefully you will see something like this.
[ 1038.060581] usb 3-1: new full speed USB device using uhci_hcd and address 2
[ 1038.220807] usb 3-1: configuration #1 chosen from 1 choice
[ 1038.370812] usbcore: registered new interface driver usbserial
[ 1038.371203] /build/buildd/linux-2.6.24/drivers/usb/serial/usb-serial.c: USB Serial support registered for generic
[ 1038.371647] usbcore: registered new interface driver usbserial_generic
[ 1038.371654] /build/buildd/linux-2.6.24/drivers/usb/serial/usb-serial.c: USB Serial Driver core
[ 1038.379409] /build/buildd/linux-2.6.24/drivers/usb/serial/usb-serial.c: USB Serial support registered for Sierra USB modem (1 port)
[ 1038.379949] /build/buildd/linux-2.6.24/drivers/usb/serial/usb-serial.c: USB Serial support registered for Sierra USB modem (3 port)
[ 1038.380525] sierra 3-1:1.0: Sierra USB modem (3 port) converter detected
[ 1038.382712] usb 3-1: Sierra USB modem (3 port) converter now attached to ttyUSB0
[ 1038.383160] usb 3-1: Sierra USB modem (3 port) converter now attached to ttyUSB1
[ 1038.383660] usb 3-1: Sierra USB modem (3 port) converter now attached to ttyUSB2
[ 1038.383986] usbcore: registered new interface driver sierra
[ 1038.383994] /build/buildd/linux-2.6.24/drivers/usb/serial/sierra.c: USB Driver for Sierra Wireless USB modems: v.1.2.5b
You can see that the modem is identified by name and its mapping is listed. The only confusing thing is that it is mapped to three devices; ttyUSB0, ttyUSB1, and ttyUSB2. I have found that pointing Kppp to the first device, /dev/ttyUSB0, will work.
As for Kppp the only thing to remember is that you don't have to fill in all of the fields when you configure the modem and the connection. The USB modem already has your user name and password. When you tell it to dial #777 it will automatically log on to the network without any input from you.
Click the Configure button to configure both the modem and a connection.
On the Accounts tab click New.
Select Manual Setup
On the Dial tab enter a name for the Connection name box.
Next to the Phone number box click on the Add box.
In the Enter a phone number box enter #777
Next to Authentication select Script-based.
Click on the OK button.
Back in the KPPP Configuration box click on the Modems tab.
Click on the New button.
In the Device tab next to Modem name enter a name for the modem.
In the Modem device select the device that you saw the modem connected to in the dmesg output. In my case it is /dev/ttyUSB0.
In the Connection speed box select 115200.
Click on the OK button.
Back in the KPPP Configuration window click on the OK button.
In the original configuration box click on the OK button.
At this point the only window open should be the KPPP startup window. Click on the Connect box. Hopefully you will soon be connected to your service provider.
I find that when I first connect to my wireless service provider Firefox doesn't recognize the connection. When I tell it to connect to a web site I get an error message saying that Firefox is running in off line mode. I click on the File menu, click on the off line item, then try to connect again. This time it always works. I haven't tried other applications yet. Just be aware that you may have to coax your applications to use this network connection.
Other people reading this should be aware that you MUST configure your 3G modem in Windows using the software that your ISP provided. This will put your user name and password into the modem. After that is done you can use the modem in Linux. I believe that my ISP set up the modem before they shipped it to me. I never had to enter my user name or password. Nevertheless I got it working in Windows before I got it running in Linux.