I don't believe anyone posted this answer, so here goes.
I had this exact problem, but found that I could open the Options dialog (Tools/Options) and set the profile (from the Profiles Tab) to a default Autodesk profile and the dictionaries were all there with American English set as the default.
Upon seeing this I realized that it was simply a path was not set.
If you then go to the Support File Search Path tab in the option Dialog, make sure you have an entry to the default Autodesk Support path, as well as any support path you may use if you customize (which we do here where I work).
I added the line ...
C:\Program Files\Autodesk\ACADM 2011\Support
... which is where the .dct files (dictionaries) are stored.
Now click Apply but don't click OK just yet.
Now go down a few lines, still on the Files tab in options, to the line that says Text Editor, Dictionary, and Font File Names.
Click 'Main Dictionary'.
If it is not populated with all dicitonaries I'd be very suprised.
Select your language and be sure to select the button 'Set Current' - now you can click 'OK'.
You should be good to go with not only your spell checker but this will also cause errors occasionally with using DDEDIT as well.
Hope this helps.
Chris.