Most probably the ubuntu will automatically detect the mobile broadband after few minutes you plug in the device.
These are the steps to add the repository and configure the mobile broadband in your ubuntu .

1) Open Terminal and switch to the root user by using this command:

/#  sudo su

2) First and foremost you have to install wvdial. Use the command to do the same:

/# sudo apt-get install wvdial   

You will be prompted whether to install or not. Press Y and complete the installation.

Now you can configure the settings for the mobile broadband.

3)  Type the following command:

/# gedit  ./etc/wvdial.conf

Screenshot from 2013-01-24 17:41:05

A new gedit window(blank) will open and with the name wvdial.conf

3)  Now add the following codes in that file and SAVE it.

 [Dialer Defaults]
Stupid Mode = 1
Inherits = Modem0
Password = mts //password as given by your broadband provider//
Username = internet@internet.mtsindia.in //username as given by your broadband provider//
Phone = #777
[Modem0]
Init1 = ATZ
SetVolume = 0
Modem = /dev/ttyUSB0 
Baud = 115200
FlowControl = Hardware (CRTSCTS)
Dial Command = ATDT

Note: Identify the USB port in which you have plugged in the device and replace the that port number in 
the 4th line of code from the bottom. The default is USB port 0
Modem = /dev/ttyUSB0

Screenshot from 2013-01-24 17:41:17

4) Mobile braodband is setup successfully.

5)  To connect to the internet, open terminal and switch to root user and type:
/# wvdial