R – sharepoint 2007 – sending alert email to operator assigned

how can i create a workflow in sharepoint designer so that whenever an item in the list is assigned to a user. the user gets email.

here is how i am doing it

in my list there is a collumn called assigned operator
while creating an item on this list the assigned operator is chosen from the active directory.
Also there is an option to chose multiple assigned operators.

i want that when an item in this list is created. the assigned operator only should get a standard email that a item has been assigned to you.

while creating a workflow using sharepoint designer , i select the option automatically start this workflow when a new item is created and press next

i chose action for step1 as send an email

i edit this message properties by

To field -> workflow lookup ->

chose the data source and the field to perform lookup on

Source : current item
Field : ( Here lies the problem – i dont see the "operator assigned" field here , all i see is the fields – created by, file type, modified by, project name and a few others but the "operator assigned" field is missing.

can you help me