I think coz your armature is linked from an external file perhaps? Am not really understanding linking/appending that much but I believe you cant EDIT an external link apart from in its own file.
You need to open a copy of the Biped rig and Append your own character. After that, you need to delete the mesh of Biped rig; select, in Object mode, your mesh and then the armature. Press Ctrl+Alt+P to parent the mesh to the armature and it’s done!
Maybe you need to resize some bones, but you need to do that before the Ctrl+Alt+P.
[]'s,
Still
PS: You need to append your character to the bipped rig file and not to link it.
Ok, let’s do it! Open the file biped_rig.blend and go to the Menu and choose save as. Save as whatever name that you want. After that, follow the steps that I have mentioned above.
I guess dubblet and copy are the same here. The steps that I have said above just works for me.