When AppleWorks is the active application, choose Edit -> Writing Tools -> Select Dictionaries from the menu. A Select Dictionaries window will appear. Click on the radio button for User Dictionary. Then below Currently Selected Dictionary, it will probably say something like 'None.' Click on Choose on the right of the window, and navigate to your MIA user dictionary, which is usually located at /Applications: AppleWorks 6: AppleWorks Essentials: Dictionaries: User Dictionary.
If you don't have that file, you can create a new user dictionary by clicking ... wait for it ... wait ... yep, click New. If that doesn't fix the problem, you may want to check the permissions of said User Dictionary to make sure that you have read/write access to the file.

