Configuring a modem as a non-admin user

You must follow these steps exactly!

In the Apple menu, go to Location and then select Home. Changing locations must be done via the Apple menu for a non-admin user. If you try to do it in System Preferences, you will be prompted for admin authentication.
Open System Preferences and click on the Network Preference Pane.
Select the Modem service on the left.
In the Configuration drop-down, select Add Configuration and enter a name. Perhaps "Earthlink" or "GWI".
Enter the phone number, account name, and password. Alternatively, leave the account name and password out, the system will prompt you when you try to connect.
You may also want to check the Show modem status in menu bar button to make it easier to connect later.
Click Apply. When prompted to authenticate, hit Cancel.
To connect, plug your phone cord into your USB modem (please see your school's tech lead for a modem if you do not have one) and click on the Connect button. Or, select Connect Internal Modem from the modem menu bar item.

Configuring a modem as an admin user

Select a location that contains the modem service (either Home or Automatic).
As an admin user, you can do this either by going to the Apple menu, highlighting Location, and selecting Home (or Automatic). Or, you can open System Preferences, select Network and use the Location drop-down menu at the top of the pane.
Next, in System Preferences, click on the Network icon and then select the modem service on the left.
Enter the account information.You should enter the Telephone number, but you may choose not to enter the username or password. We recommend against entering the password, as this causes some unexpected behavior when trying to use the configuration to connect as a non-admin. The user will be prompted to enter the username and password when the system tries to connect if you decide not to enter the information.
Check the box for "Show modem status in menu bar". This allows the non-admin user to connect without opening System Preferences.
Hit Apply and you're done!