Here are some PHP snippets to display data from custom profile fields you can set up using the core profile module in Drupal 6. In this first example I have a custom user profile field that holds the user's location with the machine readable name of profile_location. The first block of code stores the location of the user who is logged in in the variable theLocation. The second block of code displays this value.
Here is the PHP code that I use to display this code in a block on only the user's profile page. Put this code in the "Page specific visibility settings" with the "Show if PHP code returns true" option checked.