Try this link:

http://www.e-medtools.com/openmedspel.html

they have an free license to use the text of their medical dictionary. The accuracy should be good.

However I tried to manually add the dictionary to the .tlx file but then the file would not load in the AC (ie I could not pull the user dictionary) spell checker. I tried multiple techniques to place the file at certain points in the text file. Any manual editing of the file outside of the AC program renders it unusable. Either I have made a mistake in the manual editing or the spellchecker application (which is a separately licensed app by AC as I understand it) does not allow manual editing of this file.

At least you can add it to openoffice or firefox.

Geoff