You can't open dictionaries by clicking on them. First, make sure you have put them in HD > Applications > AppleWorks 6 > AppleWorks Essentials > Dictionaries. Now, you have to tell AppleWorks to use that dictionary. With a WP document open, go to Edit/Writing Tools -> Select Dictionaries... Click "User Dictionary" then click "Choose," navigate to the dictionary -> Select. Unfortunately, because there are more words in the medical dictionary than an AppleWorks 6 user dictionary can hold, you'll have to do this process to change parts.
Since there are medical terms in there I've never used, I've considered creating a new, smaller, single file dictionary, but I'm likely to leave off words someone else might need.
Peggy