For Google Analyticator 5.0+ & WordPress 2.8+:
Simply authenticate yourself with Google and you will not need your UID. Your sites will be listed in a drop down box below the authentication option.

For versions before Google Analyticator 5.0 & WordPress 2.8:
Your UID can be found on the Google Analytics website. After logging in, you should be presented with a list of sites you have registered with Google Analytics. The sites will be listed a manor similar to this:

http://www.cavemonkey50.com UA-XXXXX-X

http://ronaldheft.com UA-XXXXX-X

Your UID is the section of letters and number immediately following the URL. Copy and paste that information into Google Analyticator.