Within Teamwork Projects, there are a few ways to add a comment on an item in your project (such as a task, milestone, file, notebook or link).

Adding Comments:

To add a comment on the details page for an individual item, click on the comment box at the bottom of page:

You can also add a comment while in the list view of a particular project item (Notebooks, Messages, Tasks etc). When you hover over the project item, you will see the option to '+' Add Comment:

To add comments in the quick view overlay of a specific item, click on the comment box at the bottom of the item details area:

Formatting and Other Options:

When you click to add a comment using any of the methods above, you will see the message editor screen:

You can use the formatting toolbar to style the text in your comment, and add images or links. 

Within the comment text area, you can use the '@name' mention option to mention another person directly. That person will then automatically be added to the notify option:

Below the editor, you have the option to select who to notify of your comment, and attach files. If there isn't already a privacy setting on the item you are commenting on, you can choose who the comment should be private to.

Comment Links:

If you have lots of comments and want to link to one comment directly, you can do so by using the Permalink feature.

By clicking the link button on the left, you will get a direct link to the comment that can be shared or linked to on a Notebook or Message.
Note: Administrators can delete comments created by anyone at any time. Normal users can edit or delete their own comments within 15 minutes of adding them.

For more information see: Enabling comments on a project level