Spellbook only works on .docx files. Word documents with the .doc extension will cause the Spellbook icon to be greyed out. This is a limitation of Office/Word; Spellbook uses features of Word that are only available when .docx files are open.
To resolve this simply convert your document to a .docx file by:
Click
File→Save As...then save the file as a.docxfile
Select .docx from the dropdown and click “Save”