The NERDTree plugin creates a Vim buffer, and displays it in a Vim window. So you can use any of your normal Vim commands.
(1) Thus, if your edit window is to the right of the NERDTree window, you would use: ctrl + Wl to go to right window and ctrl + Wh to go to left window. or ctrl + w twice to toggle between the two.
(2) To go to a special file, simply search for it while in the NERDTree window:
/xxx
Hit ENTER to end the search at the line, and then ENTER again to open the file in the previous window and go to it.