How to Import an existing map to mod it.
 

Im using MOH Radiant but cant figure out for the life of me how to open an exisisting map so I can edit it.

Can anyone help? I can only find mapname.bsp which radiant wont open as it's not a file it supports. Do I have to do something simple to import it?

You can't open a bsp file in Mohradiant,
Scorpio has made a tool for viewing bsp files called mohaatools or bsp viewer, that can kinda convert it back into map format.
Craig