In the Media GUI, select the media-id's, right-click and select 'Eject'.
If you tell us your NBU and O/S version, we can point you to the correct manual. Maybe mention your tape library make & model as well?
All NBU 6.5 manuals can be found here: http://seer.entsupport.symantec.com/docs/290282.htm
Windows Admin Guide I :
http://seer.entsupport.symantec.com/docs/290203.htm
Unix Admin Guide I :
http://seer.entsupport.symantec.com/docs/290201.htm
NBU 7 manuals : http://seer.entsupport.symantec.com/docs/341274.htm
Look for the section called "Injecting and ejecting volumes" in Admin Guide I. The term in the manual for CAP (Cartridge access port) is MAP (Media access port).