If you have an address book in an email program such as Outlook Express for example, you can export the address book to a CSV file (Comma Separated Value), which is basically a text file with commas delimiting the different fields, for example: lastname, firstname, emailaddress, phone...
Export that file to a convenient location (for example, your desktop) and give it a name easy to idnetify (such as addressbook.csv).
Note: the procedure for exporting your address book varies depending on your email program, please consult the documentation of your email program to find out how to do it.
Next, you need to logon to webmail, click on the 'Organizing' menu on the left panel, and then click on 'Address Book'.
Now, click on the icon called 'Import/Export', leave the drop down box to 'Comma Separated Value' and browse to the location where you saved your address book CSV file. Click on the Next button.
Click Next again on the next page.
Select the matching fields for each value, and click 'Add Pair' each time.