To view Expenses Comments you would need to select the Expense from an Expense Report in your workspace first. Then you can go to Expense Comments option from the View Tasks navigation menu.
The Expense Comments window will open and you can view all relevant comments:
You should be able to see your comments history with the date of the comment as well as the comment itself. If you have not made any comments to the expense which you are reviewing, the Expense Comments will be blank.
You will be able to add any comments, if needed, by entering your text in the comments input area and then click Add to add the comment to the log.
As an alternative way to view the comments of an Expense, you can go to the Expense on the Expense Report directly on your workspace and view comments from there. Double click on the expense from the Expenses section and from the upper menu if that window, click on the Comments.
You can add multiple Comments, all of them will be logged and sorted in the descending order based on the created date.