Here's a short description on how to modify any profile-fields that you have created with the profile-module included in the Drupal core (at ?q=admin/user/profile). You need to know the name of the profile-field (starting with "profile_") and the category of that field (a name that you must provide when creating it). I will assume profile_test as the name and "test-category" as the category in the following, you will need to adjust these to modify fields with an other name/category.
Now, create a rule, in my case I used the trigger "user account has been created" to create a triggered rule. Then add an action and choose "PHP/Execute Custom PHP code". Now first have a look at the section "PHP-Evaluation" and find what variables of type user you have here. You will probably always $user as the acting user but may have something like $account as for my just created account (remember the trigger I used). You will need to use the right variable for your purpose here.
Now provide the following as PHP code:
$edit = array( 'profile_test' => 1 );
profile_save_profile($edit, $account, "test-category");
Remember to set $account, profile_test, and test-category according to your needs. Voila, each time the trigger gets executed, your profile-field will get set. You can of course use a variable instead of the static 1 above.