Content type column addition not showing in List modify view

You will want to create a custom field and provide your own browse UI in the rendering control template (see below). Its fairly straightforward to get started, but you need to be aware that there are multiple moving parts:

1. The field type/rendering control - this is an object that encapsulates the field definition. It is responsible for understanding how to render the field at design time (when its added to a list), at run time (when its being filled out in a form, or viewed on a list view page) and the type of value the field contains.
2. The run-time UI/rendering control template (optional) - this is the UI that is displayed when your field is collecting new values from a user on the new/edit item page.
3. The field value class (optional) - this is how you will store the value that the user enters into the field
4. The design time UI/editing control (optional) - this is the UI that a user will see when they add the custom field to a list

This is all described in the link above. There are many blog posts on creating custom fields.

My experience in these scenarios is that you need to iterate through all usages of your contenttype manually and add your field to each instance of the content type.

Using the SPContentTypeUsage class you can find the instances and loop through them. A colleague of mine has written a good blog post on the subject: http://johanleino.wordpress.com/2009/08/11/propagating-updates-to-content-types/

(I know it's for SP2007, but it applies to 2010 as well.)

Regards, Rikard