If you create a new customized workflow status, but then it doesn't appear as an option for the current piece of work you are using, you most likely need to apply it to the work type of that piece of work.

Customized workflow statuses can be used with each work type. To set up or edit these statuses first go to Settings > Workflow > Work Types. By selecting the options menu '...' to the right of the Work Type and then 'Edit' a menu will appear allowing users to select the specific set of statuses for that Work Type.

Once your changes are applied, you will be able to use that workflow status for all pieces of work with that work type.