We have installed our HDTV converter box, but when ...

First of all......Did you get the correct box for your t.v.?  Also, some older t.v.'s will need to be replaced.  In the back of the DVD player, you should have all 3 color coded wires connected....usually the 2nd set from the left if you are looking at it from the front, then have the single end plugged into the box.  You may just have them plugged into the wrong plug-ins in the back of the dvd player.  Try and see if this works and I will watch you question if you need further assistance.

God Bless,

Teresa