Based on a couple of related threads, I think you want to add an "Execute custom PHP code" action along the lines of the following (substituting in the appropriate role name in line #3):
if($account) {
$uid = $account->uid;
$role_name = 'YOUR SPECIFIC ROLE NAME GOES HERE';
$rid = db_result(db_query("SELECT rid FROM {role} WHERE name = '%s'", $role_name));
// Load user object
$account = user_load(array('uid' => 1));
// Save the user object with the new roles.
if ($account !== FALSE) {
$roles = $account->roles + array($rid => $role_name);
user_save($account, array('roles' => $roles));
watchdog('user', 'uc ca added role to Ubercart created user');