As previously indicated, there are three things you need to ensure:
• 'check grammar with spelling', under Word Options > Proofing, is checked.
• 'show readability statistics', under Word Options > Proofing, is checked.
• 'do not check spelling or grammar', under Review > Language > Set Proofing Language, is not checked, for any of the content you want to include in the stats.
The first two are global settings; the last has to be confirmed for each document.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]