After customized the toolbar, the buttons enabled, and it’s very easy to add comments, but the menu item still disabled.

The following is what I found about how to customize the toolbar from the Preview Help.

Adding text to an image

You can add text to an image to describe what’s in it or when the image was created.

After you save the image, you can’t edit, move, or delete any text you added to it. If you think you’ll need to edit the text, convert the image to a PDF document, and then add notes to the PDF document. Notes added to a PDF document can be edited after they’re saved.

To add text to an image:

1. If the Annotate pop-up menu isn’t in the toolbar, choose View > Customize Toolbar and drag the Annotate pop-up menu to the toolbar.
2. Choose Note from the Annotate pop-up menu in the toolbar.
3. Drag over the area where you want the text to appear.
4. Enter your text.

Before you save the image, you can move, resize, edit, or delete the text. First choose Text Annotation from the Annotation pop-up menu in the toolbar. Then to edit the text, double-click it. To delete the text, click it so resize handles appear, and then press Delete.