The Contact Roles related list of an Account, Case, Contract, or Opportunity displays the roles that each Contact or Person Account plays in that record. On Person Account detail pages, the Opportunity Contact Roles related list displays the Opportunities on which the Person Account is listed in the Account Name field of the Opportunity.
To edit the Contact Role picklist values you can go to:
click Your Name | Setup | Customize | Accounts | Contact Roles.
click Your Name | Setup | Customize | Contacts | Contact Roles.
click Your Name | Setup | Customize | Cases | Contact Roles.
click Your Name | Setup | Customize | Opportunities | Contact Roles.
You can then:
click "New" to add a new value
click "Reorder" to reorder the values
click "Replace" to globally replace an existing value in the picklist with a new value
click "Edit" next to an existing value to edit the value's name and, optionally, make it the default value
click "Del" next to an existing value to delete it and choose a replacement value to apply to records containing the deleted value.
The click path you choose will be determined by which object you would like the picklist value to be available for. For example, if you only need to add a value to Contact Roles for Accounts you would only need to follow the first click path and use the "New" button. If, however, you need to add a value and have it be available for Contact Roles related to all four objects you will need to follow each of the above click paths and add the value to each.