Thank you for bringing this to our attention! I can understand how important it is to access your custom fields when creating a segment for your contacts. We have received reports of customers having this same experience when they have more than 50 custom fields in their account. Your account fits this description. In these cases the custom field drop down for the Segments menu will only show the first 50 fields. As a workaround we would suggest trimming down the number of custom fields to 50 or less. I apologize for any inconvenience this may cause. I have added you to our queue to be notified when this experience is resolved.