First you need to add the device, and ensure that it can register successfully. After that you need to create a Media Resource Group (MRG) and add the Conference Bridge to this group. Then you need to create a Media Resource Group List (MRGL), and add the MRG to the MRGL.
Now just goto the phone configurations, and assign the new MRGL to the phone.
Here is a link with some info about MRG's and MRGL's
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00800c4c69.html