InfoPath Dev

I'm hoping someone here will be able to help!

First off, I'm using InfoPath 2003 and SharePoint 2007 and I'm fairly new to all of this so my knowledge is limited.

In SharePoint I have two lists.  The first list (List A) is just a basic list with a text column called Activity.  The second list (List B) has also has an Activity column that is a lookup to the Activity column in List A.

I have an InfoPath form that has a drop-down list based on the Activity column in List A.  When the user clicks the submit button, I want to add the data to List B in SharePoint.  However since Activity in List B is a lookup, I can't get it to work.  From what I've read, I have to use the ID for Activity in List A instead of the value.

My question is how do I obtain this ID?  I think I have to use the GetListItems web service but one, I can't figure out how to set-up the data connection and two, how do I use the data connection in my code once I have it set-up?

If you need any more information, let me know.  Any help would be appreciated!!

I figured out one way to do it and thought I'd update it in case someone else is trying to do this.

Set up a data connection to retrieve data from a SharePoint list.  In the last step of this set-up, when selecting fields.  Make sure that ID is one of the fields you select. 

When associating the drop-down box with the above-mentioned data connection, select the ID for the value and whatever you want for the Display name.