When you create a Custom Registration/Profile Field with s2Member (see: WordPress Dashboard s2Member General Options Registration/Profile Fields & Options Add New Field), you will need to make sure that Applicable Membership Levels is set to the same Membership Level as your Pro-From.

For instance, if you're using Level 1 Pro-Forms, set this to 1. If you want the Custom Registration Field to show up on all Pro-Forms, set this to all (the default).

You'll also want to verify that the Allow Profile Edits option is set to the appropriate setting.

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I'm still not seeing my Custom Registration Fields!

Keep in mind that Pro-Forms presented to logged-in users will not display Custom Registration Fields. If you want logged-in users to modify Custom Registration Fields, you can use the [s2Member-Profile /] shortcode. Please see: WordPress Dashboard s2Member General Options Member Profile Modifications.