I commend you on getting as far as you have.
First, let me point out that Cover Art > Add from file starts a dialog box in the directory where the music files are. So that should put you where you want to be right? That's the same thing that "locate on disk external" will do for you.
Second, you should know that commands that you just ran appear at the top of the right click menu. By default there's only one command there: The last one you ran. But you can change this to display many commands there at once. I display the last 4 I've used, which is enough for me. You can change this at:
Tools > Options > Tree & View > Advanced > Right click recent command count > (set number here)
Finally, your command for the keyboard shortcut is *very* close to correct. The problem with it is that your editor has inserted "smart quotes" around the command number and the parameter, so MC is ignoring them. Use regular quotes like this and it will work:
<Entry Key="Ctrl;K=" Command="26004" Param="-2" />
Finally, note that in this file, on a Mac, when you type "Ctrl", MC uses the <Command> key for that. You probably already knew that part.
I've tested the above line in my file and it works correctly. Oh, and you have to restart MC any time you make a change to the Resource.xml file.
Good luck.
Brian.