IMPORTANT: DO NOT CONNECT THE USB MODEM TO YOUR COMPUTER BEFORE YOU HAVE INSTALLED THE NECESSARY DRIVER
Download a copy of the driver file and configuration application from HERE
Unzip the file. Install the correct driver package for your platform (Intel or PowerPC).
Connect the USB Modem to your Mac.
Copy and paste the DataCardApp to your Applications folder:

Double click the application to launch it. The following window will be displayed:

Enter the correct APN for your service provider:
Orange
orangeinternet
Vodafone
internet
O2
mobile.o2.co.uk
T-Mobile
general.t-mobile.uk
Three
three.co.uk
Click Configure. The following window will be displayed:

Click Close.
Click on the Apple menu and select System Preferences.
Select Network.
A message will be displayed stating that new ports have been detected. Click OK. The Network Preferences window will be displayed:

Click Apply Now. In the Show menu, select HUAWEI Mobile. The following window will be displayed:

On the PP tab, enter a name for the connection in the Service Provider field. Enter the correct settings for your service provider:
Orange:
Account Name - leave blank
Password - leave blank
Telephone Number - *99#
Vodafone:
Account Name - web
Password - web
Telephone Number - *99#
O2:
Account Name - web
Password - web
Telephone Number - *99#
T-Mobile:
Account Name - leave blank
Password - leave blank
Telephone Number - *99#
Three:
Account Name - leave blank
Password - leave blank
Telephone Number - *99#
Click on the Modem tab, the following window will be displayed:

In the Modem menu, select HUAWEI Mobile Connect – 3G Modem.
Untick the option to Wait for dial tone before dialing
Click Apply Now
Open Internet Connect from the Applications menu:

Select HUAWEI Mobile.
Click the Connect button.
Addendum
With the release of the Huawei E1750 there is a native Mac client application available. It is possible to install this software for use with the E156 and E220.
The software is available for download here - ftp://ftpaccess:Brightpoint1@ftp.brightpointuk.co.uk/Technical%20Support...
Instructions on how to configure the software can be found here - http://blog.brightpointuk.co.uk/installing-huawei-e1750-usb-3g-modem-macos