Moved to proper section. Make sure you
read the getting help thread, before posting questions like these. 
Or just use the search feature to see if it's your problem has been solved before.
Generally the new users section and this halo 2 modding section is the best place to go as there are many helpful people that lurk around these forums. Unfortunatley, posting in the wrong section can just aggriavate these helpers.
Well, there could be two things here. Firstly, I've always noticed that the bsp viewer only works with the mainmenu.map shared and single player map are in the same folder as the one you're trying to edit.
If that is not it, I would recommend using an older version of entity...Why is this? because it does not request the whereabouts of the external map files. My experience is that the later versions rarely work even when I enter the maps as you have done so yourself. So
Try out this version of entity for size. I hope this one is the right one and it works out for you.